Preventing Firefox from going on top of the taskbar

"Show desktop" never worked for me, but I've noticed that if I hit F11 twice, the taskbar returns to normal. Not that that's any kind of solution, but it's easier than restarting FireFox.

I'm having the same problem too. I briefly managed to get it to behave with the "Show Desktop" command (like the original poster I used dual monitors), but this time it failed. Using Process Hacker I was able to verify that generally all Firefox windows are set to "Always On Top", even tabs that aren't active. (Miscellaneous > Windows). Toggling that doesn't work either. I also noticed another program beneath it (Pidgin) also had all its tabbed windows set to "Always On Top". After turning the flag off on all the instances AND doing a Explorer restart in Process Manager it finally behaved. I suspect it's a bug in Explorer in Windows 10.