Damn, but I'm not sure if I agree with gregp's resolution of the bug. The way I understand the changes in https://bugzilla.mozilla.org/show_bug.cgi?id=1964046, it should still use the system theme, but rendered by firefox itself. However, the current state is that it doesn't follow the system theme anymore :/
EDIT: I just saw this comment: https://lemmy.world/comment/17957836
And yep, that's correct. I'm also using the Papirus icon theme, when I change the theme to breeze or something else, the buttons in firefox titlebar also reflect this change after a restart. So Firefox is now using the window-{maximize,minimize,close,....}-symbolic
icons from the icon theme and not from the window decorations setting.
This works, but it's the same as disabling
browser.tabs.inTitlebar
. The result is a separate titlebar above the tab bar.