There’s a bunch of different ways you can customize it.
- Default right click -> customize toolbars fore simple rearranging of toolbars, density setting and stuff
- Changes made in about:config (such as “browser.tabs.tabMinWidth”)
- And the most powerful but difficult, userChrome.css. The UI of Firefox is actually defined by CSS. More info: https://www.userchrome.org/
EDIT: Oh and there are of course addons and themes too
What exactly does this mean for everyday Linux usage?