bttt
Shut down you PC and restart immediatley
It didn't terminate properly - it's still running, even though it looks like it's quit. Pull up the task manager, right-click on firefox.exe, and select "End Process". It should restart normally after that. Alternately, log off and back on, or reboot to kill it and restart normally.
What has happened is that Firefox crashed. It leaves a module running in the background in Windows. You can go to your task manager and shutdown the module, but a restart usually is easier.