Thanks! Firefox uses the default GTK theme (Rosé Pine Dawn), but no custom userChrome.css is set (though, I’ll most probably change that).

In sowm (maybe in some other window managers as well) I can press F11 in Firefox and it will hide all the UI stuff and show only the web-site I’m on, without actually going fullscreen (which is done by pressing Super + F in my setup). So, generally I don’t see any of Firefox’s UI at all.