If you have a theme enabled that isn't the Light or Dark one. The window controls behave erratically. Some times they fully dissappear, some times partially appear. This started to happen on version 84.0b3
I noticed that it also happens when I enable the Firefox Apenglow Theme.
I noticed the issue has to do with the hardware acceleration on version 84.0b3. If I disable it, the window controls show correctly on any theme, but if it is enabled the window controls become unstable on any theme except dark and light ones.
Issue does not happen in 84.0b2 and appeared in 84.0b3.