PSA: don’t overwrite your firefox pref.js unless you want to lose all settings you’ve customized yourself. If you go to about:config and set ‘browser.uidensity’ to ‘1’ it will make things more compact. Backup your pref.js first if you intend to try theirs out!
*Firefox Edit: prefs.js
Replace the existing file in your Firefox profile directory.
CLOSE FIREFOX before doing so, otherwise it’ll be overwritten by Firefox prematurely
*This Firefox edit is pulled from the Feren OS configuration.