Firefox attains a not responding state after prolonged use at near maximum OS memory usage (this computer has only 8GB of RAM). Normally this is fine and I would simply restart the browser. Allowing Windows to terminate the non-responsive Firefox closes the windows, but almost all tab processes remain loaded in memory. This prevents Firefox from opening again, because it can't start while the old session is running, apparently. These threads can be manually killed in task manager—all except for one. The last thread always fails with an access denied error. I also used an elevated command prompt to try ending all the processes; every thread was detected, even the forbidden one, yet it still failed to close (with a "there is no running instance of the task" notice rather than access denied): C:\Users\amalesevic>taskkill /f /im firefox.exe ERROR: The process "firefox.exe" with PID 2472 could not be terminated. Reason: There is no running instance of the task. SUCCESS: The process "firefox.exe" with PID 11100 has been terminated. SUCCESS: The process "firefox.exe" with PID 19188 has been terminated. This is on my work PC and there may be some administrator-configured access issue wrongly preventing certain threads from closing. I'm really not sure.