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30Jan2019

Firefox: tabs on bottom (again)

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64 hits Updated: 31 January 2019

Firefox Quantum (Firefox 65) uses tabs on top

How to place tabs on bottom

This article shows you how to change Firefox’s appearance from tabs on top (where the tabs appear above the URL address bar) to display the tabs below the URL address bar.

Before the release of Firefox Quantum (i.e. FF v57), it was possible to change the location of the tabs by changing a browser parameter setting.  The setting was

browser.tabs.onTop = false

FF  still retains the parameter but its functionality has been disabled; browser tabs remain on top (above the URL address bar) regardless of this parameter’s value.  We will see how it’s possible to change the location of the tabs.My earlier article, Firefox: tabs on bottom, discusses an older version of FF.  This article updates that advice following the release of FF65.

Tabs on top vs. on bottom

The following image shows the difference between tabs on top, which is the new behaviour for FF (shown on the left), and tabs on the bottom (shown on the right).  It is a matter of personal choice whether people prefer one style over the other.

ff57 tabs

To change the behaviour so that the tabs appear on the bottom we will need to write a small piece of CSS and store the file on your computer.

Creating the CSS file

Locating your Firefox browser profile

Press the keyboard characters ⊞ Win + R to bring up the Run dialog.

FF profiles windowsType %APPDATA%\Mozilla\Firefox\Profiles in the text box and press OK.  This will open Windows Explorer (see the image on the right; click to enlarge it).

You should see a folder with the name xxxxxxxx.default (or possibly xxxxxxxx.default-nnnnnnnnnnnnn) where xxxxxxxx are randomly-generated characters and nnnnnnnnnnnnn is the epoch timestamp representing the date/time when the folder was created.

ff profile chromeThe next thing to do is to navigate within this just-named, folder.  Look for a sub-folder named /chrome (see the image on the right for more information).  Navigate into that folder.

It is highly probable that there will be no files within the chrome folder.  The next thing we will do is create a new file named userChrome.css.  Open an instance of Windows Notepad, select the code segment below and paste this text into Notepad.

/* Combined userChrome.css taken from aris-t2's code /*
/* Firefox Quantum userChrome.css tweaks ************************************************/
/* Github: https://github.com/aris-t2/customcssforfx ************************************/
/****************************************************************************************/

:root {
--colored_menubar_background_image: linear-gradient(#f9f9fa,#f9f9fa);
--general_toolbar_color_toolbars: linear-gradient(#f9f9fa,#f9f9fa);
--general_toolbar_color_navbar: linear-gradient(#f9f9fa,#f9f9fa);
--tabs_toolbar_color_tabs_not_on_top: linear-gradient(#f9f9fa,#f9f9fa);
--tabs_toolbar_border-tnot_normal_mode_size: 1px;
--tabs_toolbar_border-tnot_normal_mode: var(--tabs-border-color);
--classic_squared_tabs_tab_text_color: black;
--classic_squared_tabs_new_tab_icon_color: black;
--classic_squared_tabs_tab_text_shadow: transparent;
--tab-min-height_tnot: 32px;
}

:root[uidensity=compact] {
--tab-min-height_tnot: 28px;
}

:root[uidensity=touch] {
--tab-min-height_tnot: 40px;
}

/* tabs toolbar adjustment */
#main-window[tabsintitlebar] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#TabsToolbar {
-moz-padding-start: 2px !important;
}

#main-window[tabsintitlebar] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar:-moz-lwtheme,
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar:-moz-lwtheme,
#main-window[uidensity=compact][tabsintitlebar] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar:-moz-lwtheme,
#main-window[uidensity=compact][tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar:-moz-lwtheme,
#TabsToolbar:-moz-lwtheme,
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[sizemode="maximized"] #TabsToolbar,
#main-window[tabsintitlebar]:not([style*='--lwt-header-image']):-moz-lwtheme #navigator-toolbox #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[tabsintitlebar][sizemode="maximized"]:not([style*='--lwt-header-image']):-moz-lwtheme #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar]:not([style*='--lwt-header-image']):-moz-lwtheme #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="maximized"]:not([style*='--lwt-header-image']):-moz-lwtheme #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[tabsintitlebar]:not([style*='--lwt-header-image']):-moz-lwtheme #TabsToolbar {
-moz-padding-start: 0px !important;
}

#main-window[tabsintitlebar][sizemode="normal"]:not([sizemode="fullscreen"]) #TabsToolbar {
margin-top: 0px !important;
}

#main-window[tabsintitlebar][sizemode="normal"][chromehidden~="menubar"] #TabsToolbar,
#main-window[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"][inactive] + #TabsToolbar,
#main-window[tabsintitlebar][sizemode="maximized"][chromehidden~="menubar"] #TabsToolbar,
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive] + #TabsToolbar {
padding-top: 0px !important;
}

#main-window[tabsintitlebar] #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar,
#main-window[tabsintitlebar] #toolbar-menubar[autohide="false"] ~ #TabsToolbar{
margin-top: 0px !important;
}

/* tab title in titlebar support */
#main-window[tabsintitlebar][sizemode="normal"] #navigator-toolbox #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="normal"] #navigator-toolbox #TabsToolbar,
#main-window[tabsintitlebar][sizemode="maximized"] #navigator-toolbox #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="maximized"] #navigator-toolbox #TabsToolbar,
#main-window[tabsintitlebar][sizemode="normal"] #navigator-toolbox #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="normal"] #navigator-toolbox #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[tabsintitlebar][sizemode="maximized"] #navigator-toolbox #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[uidensity=compact][tabsintitlebar][sizemode="maximized"] #navigator-toolbox #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar{
margin-top: 0px !important;
}

/* titlebar button support */
#main-window[tabsintitlebar] #toolbar-menubar[autohide="true"][inactive="true"] ~ #nav-bar{
margin-top: 2px !important;
}

/* fix for application/hamburger button in titlebar */
#main-window[tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="true"][inactive="true"] ~ #nav-bar #PanelUI-button {
visibility: collapse !important;
}

#main-window[inDOMFullscreen="true"] #TabsToolbar {
opacity: 0 !important;
}

/* tab height */
#tabbrowser-tabs,
#tabbrowser-tabs > .tabbrowser-arrowscrollbox,
.tabbrowser-tabs[positionpinnedtabs] > .tabbrowser-tab[pinned] {
min-height: var(--tab-min-height_tnot) !important;
}

/* toolbar order (start) ************************************/
#print-preview-toolbar,
#printedit-toolbar,
#titlebar {
-moz-box-ordinal-group: 0 !important;
}
#navigator-toolbox #toolbar-menubar {
-moz-box-ordinal-group: 1 !important;
}
/* navigation toolbar */
#navigator-toolbox #nav-bar {
-moz-box-ordinal-group: 2 !important;
}
/* bookmarks toolbar */
#navigator-toolbox #PersonalToolbar {
-moz-box-ordinal-group: 3 !important;
}
/* 3rd party toolbars */
#navigator-toolbox toolbar {
-moz-box-ordinal-group: 10 !important;
}
/* tabs toolbar */
#navigator-toolbox #TabsToolbar {
-moz-box-ordinal-group: 100 !important;
}
/* toolbar order (end) **************************************/

/* toolbar colors */
#main-window:not([tabsintitlebar]) #toolbar-menubar:not(:-moz-lwtheme) {
background-image: var(--colored_menubar_background_image) !important;
border: 0px !important;
margin-bottom: -1px !important;
}

#nav-bar:not(:-moz-lwtheme) {
-moz-appearance: none !important;
background: var(--general_toolbar_color_navbar, inherit) !important;
}

toolbar:not(#TabsToolbar):not(#toolbar-menubar):not(#nav-bar):not(:-moz-lwtheme) {
-moz-appearance: none !important;
background: var(--general_toolbar_color_toolbars, inherit) !important;
}
#TabsToolbar:not(:-moz-lwtheme){
-moz-appearance: none !important;
background-image: var(--tabs_toolbar_color_tabs_not_on_top) !important;
}

/* remove color overlay for lw-themes */
#main-window[style*='--lwt-header-image'] :-moz-any(#nav-bar,#PersonalToolbar,#TabsToolbar):-moz-lwtheme{
background: unset !important;
}

/* adjust compact themes background color */
#main-window:not([style*='--lwt-header-image']) #TabsToolbar:-moz-lwtheme {
-moz-appearance: none !important;
background-image: linear-gradient(var(--toolbar-bgcolor),var(--toolbar-bgcolor)) !important;
}

#main-window:not(:-moz-lwtheme) #TabsToolbar :-moz-any(.tabs-newtab-button,.scrollbutton-up,.scrollbutton-down),
#main-window:not(:-moz-lwtheme) #TabsToolbar > toolbarbutton,
#main-window:not([style*='--lwt-header-image']):-moz-lwtheme:-moz-lwtheme-darktext #TabsToolbar :-moz-any(.tabs-newtab-button,.scrollbutton-up,.scrollbutton-down),
#main-window:not([style*='--lwt-header-image']):-moz-lwtheme:-moz-lwtheme-darktext #TabsToolbar > toolbarbutton {
fill: var(--classic_squared_tabs_new_tab_icon_color) !important;
color: var(--classic_squared_tabs_new_tab_icon_color) !important;
}

/* toolbar borders */
#main-window #navigator-toolbox::after {
opacity: 0 !important;
}

#TabsToolbar{
margin-bottom: 0px !important;
border-bottom: 1px solid #5f7181 !important;
}

#main-window[sizemode="normal"] #navigator-toolbox :-moz-any(#nav-bar,#PersonalToolbar,#TabsToolbar) {
border-left: var(--tabs_toolbar_border-tnot_normal_mode_size) solid var(--tabs_toolbar_border-tnot_normal_mode) !important;
border-right: var(--tabs_toolbar_border-tnot_normal_mode_size) solid var(--tabs_toolbar_border-tnot_normal_mode) !important;
background-clip: padding-box !important;
}

#main-window[sizemode="maximized"] #navigator-toolbox :-moz-any(#nav-bar,#PersonalToolbar,#TabsToolbar) {
border-left: 0px !important;
border-right: 0px !important;
}

#main-window[style*='--lwt-header-image'][sizemode="normal"] #navigator-toolbox :-moz-any(#nav-bar,#PersonalToolbar,#TabsToolbar):-moz-lwtheme,
#main-window[style*='--lwt-header-image'][sizemode="maximized"] #navigator-toolbox :-moz-any(#nav-bar,#PersonalToolbar,#TabsToolbar):-moz-lwtheme {
border-top: 0px !important;
border-left: 0px !important;
border-right: 0px !important;
}

#main-window:not([style*='--lwt-header-image']):-moz-lwtheme-brighttext #TabsToolbar:-moz-lwtheme {
border-bottom: 1px solid #323234 !important;
}

#main-window:not([style*='--lwt-header-image']):-moz-lwtheme:-moz-lwtheme-brighttext #TabsToolbar:-moz-lwtheme {
border-bottom: 1px solid #323234 !important;
}

#main-window:not([sizemode="fullscreen"])[tabsintitlebar] #TabsToolbar{
-moz-margin-end: 0px !important;
}

#main-window[tabsintitlebar] #TabsToolbar .titlebar-placeholder{
visibility: collapse !important;
}

#main-window[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"][inactive="true"] {
margin-top: 21px !important;
}
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] {
margin-top: 20px !important;
}

#tabbrowser-tabs .tab-drop-indicator {
margin-bottom: 0px !important;
}

#PersonalToolbar:-moz-lwtheme,
#nav-bar:-moz-lwtheme {
background-image: none !important;
box-shadow: none !important;
border-top: none !important;
border-bottom: none !important;
}

/* remove tab fog */
#TabsToolbar:not(:-moz-lwtheme),
#TabsToolbar:not(:-moz-lwtheme)::before,
#TabsToolbar:not(:-moz-lwtheme)::after {
box-shadow: unset !important;
}

/* remove 'dragging tab' margin/padding nonsense */
#TabsToolbar[movingtab] {
padding-bottom: unset !important;
}

#TabsToolbar[movingtab] > .tabbrowser-tabs {
padding-bottom: unset !important;
margin-bottom: unset !important;
}

#TabsToolbar[movingtab] + #nav-bar {
margin-top: unset !important;
}

/* scroll buttons */
#TabsToolbar:not(:-moz-lwtheme) #alltabs-button,
#TabsToolbar:not(:-moz-lwtheme) .tabbrowser-arrowscrollbox > .scrollbutton-up,
#TabsToolbar:not(:-moz-lwtheme) .tabbrowser-arrowscrollbox > .scrollbutton-down {
fill: black !important;
}

/* Fx58+ titlebar placeholders */
#TabsToolbar *[type="pre-tabs"],
#TabsToolbar *[type="post-tabs"] {
display: none !important;
}

@media (-moz-os-version: windows-win10) {
#main-window[tabsintitlebar][sizemode="normal"] #toolbar-menubar[autohide="true"][inactive="true"] {
margin-top: 30px !important;
}
#main-window[tabsintitlebar][sizemode="maximized"] #toolbar-menubar[autohide="true"][inactive="true"] {
margin-top: 30px !important;
}
}

@media (-moz-windows-classic) {

/* tweaks for Windows Classic theme */
#nav-bar:not(:-moz-lwtheme),
#PersonalToolbar:not(:-moz-lwtheme),
#main-window:not([tabsintitlebar]) #TabsToolbar:not(:-moz-lwtheme) {
border-left: unset !important;
border-right: unset !important;
background: unset !important;
}

#main-window[sizemode="normal"] #navigator-toolbox :-moz-any(#nav-bar,#PersonalToolbar,#TabsToolbar) {
border-left: 0px !important;
border-right: 0px !important;
}

#main-window:not([tabsintitlebar]) #toolbar-menubar:not(:-moz-lwtheme) {
background: unset !important;
box-shadow: unset !important;
border-bottom: unset !important;
}

#main-window:not([tabsintitlebar]):not(:-moz-lwtheme) #navigator-toolbox,
#main-window:not([tabsintitlebar]):not(:-moz-lwtheme) #nav-bar,
#main-window:not([tabsintitlebar]):not(:-moz-lwtheme) #navigator-toolbox::before,
#main-window:not([tabsintitlebar]):not(:-moz-lwtheme) #nav-bar::before,
#main-window:not([tabsintitlebar]):not(:-moz-lwtheme) #navigator-toolbox::after,
#main-window:not([tabsintitlebar]):not(:-moz-lwtheme) #nav-bar::after {
background: unset !important;
box-shadow: unset !important;
border: unset !important;
border-image: unset !important;
}

#main-window:not([tabsintitlebar]) toolbar:not(#TabsToolbar):not(#toolbar-menubar):not(#nav-bar):not(:-moz-lwtheme) {
background: unset !important;
}

}

/* tab text colors */
#TabsToolbar:not(:-moz-lwtheme) .tabbrowser-tab {
color: var(--classic_squared_tabs_tab_text_color) !important;
text-shadow: 1px 1px 1px var(--classic_squared_tabs_tab_text_shadow) !important;
}

/* fix Windows 7s default tab background (remove dark purple bg color)*/
@media (-moz-os-version: windows-win7) {
@media (-moz-windows-default-theme) {
.tabbrowser-tab:not([selected]):not(:hover):not(:-moz-lwtheme) {
background-color: #e3e4e6 !important;
}
.tabbrowser-tab:not([selected]):hover:not(:-moz-lwtheme) {
background-color: #cccdcf !important;
}

}
}

/* Simple Close Tab button */
#TabsToolbar:not(:-moz-lwtheme) #_fe8cab45-aaee-45f8-8765-781e0ebea300_-browser-action {
--webextension-toolbar-image-light: url("moz-extension://7904448c-2319-4f6f-aac9-a1f701258098/icons/Close-dark.svg") !important;
}

/* close tab button */
#TabsToolbar:not(:-moz-lwtheme) #closetab_fmarin_com-browser-action {
--webextension-toolbar-image-light: url("moz-extension://31006e48-8e00-476f-8ea9-697e40934b02/icons/icon.svg") !important;
--webextension-toolbar-image-2x-light: url("moz-extension://31006e48-8e00-476f-8ea9-697e40934b02/icons/icon.svg") !important;
}

/* close window button */
#TabsToolbar:not(:-moz-lwtheme) #close-window-button_nanpuyue_com-browser-action {
--webextension-toolbar-image-light: url("moz-extension://c914bd13-793f-4a2d-ab66-c655a3d75c52/icons/icon.svg") !important;
--webextension-toolbar-image-2x-light: url("moz-extension://c914bd13-793f-4a2d-ab66-c655a3d75c52/icons/icon.svg") !important;
}

#main-window:not([chromehidden*="toolbar"]) #navigator-toolbox {
padding-bottom: calc(1px + var(--tab-min-height_tnot)) !important;
}

#TabsToolbar {
position: absolute !important;
bottom: 0 !important;
width: 100vw !important;
}

#tabbrowser-tabs {
width: 100vw !important;
}

/* make sure button icon colors set correctly */
#main-window:not(:-moz-lwtheme) #TabsToolbar-customization-target toolbarbutton .toolbarbutton-icon,
#main-window:not([style*='--lwt-header-image']):-moz-lwtheme:-moz-lwtheme-darktext #TabsToolbar-customization-target toolbarbutton .toolbarbutton-icon {
fill: var(--classic_squared_tabs_new_tab_icon_color) !important;
color: var(--classic_squared_tabs_new_tab_icon_color) !important;
}

/* move caption buttons to windows top right position */
#main-window[tabsintitlebar]:not([inDOMFullscreen="true"]) #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
position: fixed !important;
right: 0 !important;
visibility: visible !important;
display: block !important;
}

/* caption button position in maximized mode after moving to the top */
#main-window[tabsintitlebar]:not([inDOMFullscreen="true"]) #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container,
#main-window[tabsintitlebar][sizemode="maximized"]:not([inDOMFullscreen="true"]) #TabsToolbar .titlebar-buttonbox-container {
top: -14px !important;
}


/* Windows */
@media (-moz-os-version:windows-win10), (-moz-os-version:windows-win8), (-moz-os-version:windows-win7) {
#main-window[tabsintitlebar]:not([inDOMFullscreen="true"]) #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
top: 1px !important;
}
#main-window[tabsintitlebar][sizemode="maximized"]:not([inDOMFullscreen="true"]) #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
top: 8px !important;
}
}

/* Windows 7 extra tweaks */
@media (-moz-os-version: windows-win7) {
@media all and (-moz-windows-compositor) {
#main-window[tabsintitlebar]:not([inDOMFullscreen="true"]) #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
display: none !important;
}
}
@media not all and (-moz-windows-compositor) {
#main-window[tabsintitlebar]:not([inDOMFullscreen="true"]) #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
top: -2px !important;
}
#main-window[tabsintitlebar][sizemode="maximized"]:not([inDOMFullscreen="true"]) #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container {
top: 0px !important;
}
#main-window[tabsintitlebar]:not([inDOMFullscreen="true"]) #toolbar-menubar[autohide="true"]:not([inactive="true"]) ~ #TabsToolbar .titlebar-buttonbox-container {
display: none !important;
}
}
}

/* Windows 10 extra tweaks */
@media (-moz-os-version: windows-win10) {
#main-window:not([tabsintitlebar]) menubar > menu:not(:-moz-lwtheme):hover:not([disabled="true"]),
#main-window:not([tabsintitlebar]) menubar > menu:not(:-moz-lwtheme)[_moz-menuactive="true"]:not([disabled="true"]) {
background-color: Highlight !important;
color: HighlightText !important;
}
}

/* tweaks for fullscreen mode */
#main-window[tabsintitlebar][sizemode="fullscreen"] #toolbar-menubar[autohide="true"] ~ #TabsToolbar .titlebar-buttonbox-container,
#main-window[tabsintitlebar][sizemode="fullscreen"] #navigator-toolbox #PanelUI-button,
#main-window #TabsToolbar #window-controls {
display: none !important;
}

/* hide non-required items */
#TabsToolbar .private-browsing-indicator,
#TabsToolbar #window-controls,
#TabsToolbar *[type="caption-buttons"],
#TabsToolbar *[type="pre-tabs"],
#TabsToolbar *[type="post-tabs"] {
display: none !important;
}

/* adjust tabs toolbar padding */
#main-window:-moz-any([sizemode="normal"],[sizemode="maximized"],[sizemode="fullscreen"]) #navigator-toolbox #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window[tabsintitlebar]:-moz-any([sizemode="normal"],[sizemode="maximized"],[sizemode="fullscreen"]) #navigator-toolbox #toolbar-menubar[autohide="true"][inactive="true"] ~ #TabsToolbar,
#main-window:-moz-any([sizemode="normal"],[sizemode="maximized"],[sizemode="fullscreen"]) #navigator-toolbox #TabsToolbar,
#main-window[tabsintitlebar]:-moz-any([sizemode="normal"],[sizemode="maximized"],[sizemode="fullscreen"]) #navigator-toolbox #TabsToolbar {
-moz-padding-start: 0px !important;
-moz-margin-start: 0px !important;
}

/* hide line above navigation toolbar appearing in some cases */
#main-window:not([tabsintitlebar]) #nav-bar,
#main-window:not([tabsintitlebar]) #navigator-toolbox {
border-top: 0 !important;
box-shadow: unset !important;
}

/* disable Mozillas tab jumping nonsense when moving tabs */
#navigator-toolbox[movingtab] > #titlebar > #TabsToolbar {
padding-bottom: unset !important;
}
#navigator-toolbox[movingtab] #tabbrowser-tabs {
padding-bottom: unset !important;
margin-bottom: unset !important;
}
#navigator-toolbox[movingtab] > #nav-bar {
margin-top: unset !important;
}

/* size of new tab tabs '+' icon */
.tabs-newtab-button .toolbarbutton-icon {
padding: 0px !important;
margin: 0px !important;
width: 18px !important;
height: 18px !important;
}

Save the file with the name userChrome.css.

Close all existing instances of Firefox—i.e. close all windows that are running FF—and then open a new instance; effectively “restart” Firefox.  You should now see the tabs on the bottom.

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# A. Burnette 15-Feb-2019 01:57
I was able to use your 2018 userChrome.css file to move the tabs to the bottom on firefox under ubuntu 18.04. Will your 2019 .css file work under ubuntu? In 2018 I was somehow able to install the .css file in a new /chrome directory in the default profile. Now I can't remember how to find the default profile.
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# sozzled 15-Feb-2019 06:48
Thank you for your question. Unfortunately I don't use Ubuntu on my personal computer for web browsing and so I am not able to answer your question. The article I've written relates to how to modify the Firefox CSS for Windows-based systems. I wish you well in finding the answer.
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# A. Burnette 16-Feb-2019 07:05
I finally found the /chrome directory, and copied the 2019 userChrome.css into it. I now have the tabs back on the bottom in firefox 65, running under ubuntu 18.04. Thanks for making the updated .css file available.
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# sozzled 16-Feb-2019 07:39
Thank you, too, for your comments.
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