See the screenshots:
Created attachment 112817 [details] Firefox-nightly under Gnome
Created attachment 112818 [details] Firefox-nightly under Plasma
Theme is the same (adwaita-dark + elementary icons). I would say that it looks correctly on Gnome, on the other hand the frame on Plasma kind of replaces shadows.
Why do you report issues about Firefox to a window manager? Please report to Firefox.
(In reply to Martin Flöser from comment #4) > Why do you report issues about Firefox to a window manager? Please report to > Firefox. I will, I thought Plasma is also at least a bit at fault but you are right.