Here is a Youtube video that show the problem: https://youtu.be/AGIN-8Md2vQ
1. Play a video in fullscreen from any website (I tested on Youtube, dailymotion, uptobox, Amazon Prime Video)
2. Returns to windowed mode.
3. Change tab, resize the Firefox window. As you can see the titlebar is bugged.
I didn't change anything in Firefox. The performance setting is set to the recommended settings.
I have to unload WindowBlinds and load it again to recover a normal titlebar or restart Firefox.
I didn't have this problem with WindowBlinds 10.84
Reproducible using any WB skin.
Windows 10 Enterprise (19041.vb_release.191206-1406)
WindowBlinds 10.87 (062 - Windows 10 Edition) - 64 bit OS