This happens in the last few days with my default browser, I click "minimize" and suddenly everything is gone, also the taskbar button is gone. I have no "Quit Firefox" warning enabled, and I don't know if it would be shown (I'll enable it for some days). Firefox is still fully present in the taskmanager however and I have to terminate the process in order to go on. Good chance that one of my extensions has become incompatible and is causing this now, but I'm not sure and hence I'm filing this bug. I hope I have additional information later.
I don't see this anymore.