Sometimes it disappears from my Windows taskbar. It’s possible that this occurs only when I have multiple Firefox windows open. I can retrieve the windows by either minimizing windows until I get to the one I want, or the simpler method of alt-tab. They’re always in alt-tab.
Sometimes I’ll be able to get one window back onto the toolbar but not all of them.
Yeah, this is a legitimate bug, unlike my typical Firefox gripes, which tend to be about omitted or clunky features.