Firefox v69+ no longer loads the file “userChrome.css” or “userContent.css” by default. Users who wish to customize Firefox (have the tabs on the bottom, for example) by using one of these files can set the “toolkit.legacyUserProfileCustomizations.stylesheets” preference to "true" to restore this ability. Follow the steps below:
1) Download this file “userChrome.css.zip” to your desktop temporarily. Unzip it if necessary. It works for both Macs and Windows based PC's
2) Navigate your file system to Firefox’s “Profiles” folder. On a Mac, the path is Users / AccountNameXXXX / Library / Application Support / Firefox / Profiles / XXXXX.default-release-X
It might be easier to use the Search function in the upper right corner of any Finder window. Search for Profiles.
3) Once you’re at the Profiles folder’s XXXXX.default-release-X folder, you may need to create a new folder inside it (Command + Shift + N) and name it chrome (no caps).
4) Drag the “userChrome.css” file to the new chrome folder.
5) Open Firefox and in the Location or URL box at the top of a page, enter: about:config , hit Enter, and then scroll near the bottom to “toolkit.legacyUserProfileCustomizations.stylesheets”, in a column to the right double-click on "false" to change it to "true".
6) Quit Firefox and reopen it, the “userChrome.css” file should have loaded and you have tabs on the bottom.
Note: I am not the author of the userChrome.css. The author has lots of information on tweaking Firefox at this site: github.com/aris-t2/customcssforfx
Original copy of tabs on bottom code:
Want to know more, visit: www.userchrome.org/how-create-userchrome-css.html