Custom userChrome for Firefox to minimize the space used for url and tab bar:
Working as of Nov. 6th 2022: Firefox 107.0b8 on Fedora 36 and 106.0 on Manjaro.
Go to about:config and set toolkit.legacyUserProfileCustomizations.stylesheets to true
browser.compactmode.show should also be set to true and enabled (see: Ghacks article)
After doing this, Firefox will try to load userChrome.css from the chrome folder inside your FF profile.
Use about:support to find the path to your profile folder. Example: /home/USERNAME/.mozilla/firefox/abc1234.dev-edition-default/chrome/
Feel free to use a theme of your choice with these styles.
Tested with the default themes (light, dark, alpenglow) as well as a Nord theme (used in the screenshot above, source).
This stylesheet is based on: