Created attachment 851158 [details]
Screenshot of Mozilla Firefox in fullscreen mode showing Microsoft style
titlebar-button icons.

If you open any page in fullscreen mode (F11) and move the pointer to the top
of the screen, so that the titlebar appears again, Firefox uses Microsoft
Windows titlebar-button icons instead of the ones provided by the selected icon
theme like it does when not Firefox is not in fullscreen.

Steps to reproduce:
- Open Mozilla Firefox (in my case 90.0)
- Open any page
- Press F11 on your keyboard
- Move pointer to top of screen

I expect Firefox to use the titlebar-button icons provided by the selected icon
theme (gsettings: org.gnome.desktop.interface icon-theme).

This occurs with MozillaFirefox-90.0-1.1.x86_64 on openSUSE Tumbleweed 20210718