You can change the density of Firefox's UI in about:config by setting "browser.uidensity" between 0 to 2, and it applies immediately! ( lemmy.world )
Crossposted another Reddit user's post: https://reddit.com/r/firefox/comments/1dso5sz/easy_trick_to_make_firefox_compact_1_in_firefoxs/
![](https://mbin.grits.dev/media/cache/resolve/entry_thumb/d9/da/d9da14538977c1c513d7eaf85d061596dc905863c978873cfc97aee27360c857.png)