Do you mean the Tab bar just below the Navigation Toolbar and Bookmarks Toolbar or completely at the bottom of the window?

What code do you currently use, because there hasn't been a change to how this works?

Method from earlier thread:

(A) Create a new chrome folder in your profile folder

The following article has the detailed steps for that (#1, #2, and I recommend #3)

https://www.userchrome.org/how-create-userchrome-css.html

I also created video for both Windows and Mac, on that same page.

(B) Set your OS to show file extensions

See Step #4 in the above article.

(C) Download the following file and move it into your new chrome folder

Firefox 57-64 (Windows and Mac): https://www.userchrome.org/samples/userChrome-tabs_on_bottom.css

Firefox 65+ (Windows): https://www.userchrome.org/samples/userChrome-tabs_on_bottom_Fx65.css

Firefox 65+ (Mac): See: Custom CSS tweaks for Firefox Quantum.

(D) Rename the file to just userChrome.css

Windows: You can right-click the file, then Rename, then remove the -tabs_on_bottom or -tabs_on_bottom_Fx65 part from the file name so you end up with exactly userChrome.css and then click away to complete the edit.

Mac: To make the name of a file editable, you can select the file in Finder and press Return, or you can "force-click" it if you have one of the new touchpads. Remove the -tabs_on_bottom part from the file name so you end up with exactly userChrome.css and then click away to complete the edit.

The next time you exit Firefox and start it up again, it should discover that file and apply the rules.

New Comment:

It was suggested in another thread to use a much higher value for this rule:

#TabsToolbar { /* tab bar */
  -moz-box-ordinal-group: 3 !important;
  padding-top: 0 !important;
}

For example:

#TabsToolbar { /* tab bar */
  -moz-box-ordinal-group: 100 !important;
  padding-top: 0 !important;
}

I haven't found that necessary when I test the style rule on my Firefox (on Windows 7) but perhaps it is needed on some systems.

OK thought the problem was file folder was capitalized "Chrome" & not "chrome", simple typo. It started working after it was renamed. BTW, both settings in #TabsToolbar of "3" 0r "100" works for me :

#TabsToolbar { /* tab bar */
  -moz-box-ordinal-group: 100 !important;
  padding-top: 0 !important;
}

Tested again w/ both capital & lower case "chrome" file name and both worked, however, the only other thing I did was delete the "!important" text but left semicolon, (?) who knows? it's working now. Thanks to ALL for this workaround. Glad it still is working.

running 62.02 and have no issues .It still works fine

Awesome! Works perfectly on Firefox 63.0.1

Why doesn't Firefox simply have a menu selection? It would save so many of us so much time and trouble.