I notice now (and should I have noticed earlier) that Firefox ignores the various themes used in Budgie.
I’m mainly talking about minimise, maximise window buttons.
Is it just my setup or a common issue?
I have never encountered this problem under Ubuntu. Which version of Firefox are you using? A version from the official Ubuntu package sources or a direct download from Mozilla’s website? Or maybe even a snap or flatpak version?
For instance if you use FireFox as a snap,
make sure you also have your favorite gtk-theme installed as a snap.
And connect this theme to your snap app’s.
Many themes are available in snap - but not all.