Menu bar, with the window active:
I get that the thing's basically an 'abandoned leftover' from when UI design used to be much more sane (and honestly, if it weren't for modern websites breaking in anything relatively recent I'd main SeaMonkey), but it's been like this since Australis was introduced if I'm not wrong - and I have never managed to find any solution to the issue. Presumably because no one uses the menu bar (or even the title bar nowadays from what it seems).
Running Windows 7, though the issue can be replicated on any Firefox going back to 29, so I guess the problem can be applied to Vista too (not sure about other OSes using non-Aero, such as XP, but it does seem to still linger around even when the transparency effects are turned off, so it might actually just be a Vista/7 DWM issue).
Currently running FF91.5.0ESR, x64. Been a user since the 3.5 days, btw. Hi.