Sometimes if I close firefox then try to open it again it will tell me the program is still running even though it is not listed as running in task manager application tab. I click on the process tab from the task manager and shut the firefox.exe down. Then open firefox again and it opens.
I’ve seen that symptom too, sounds like the same problem manifesting itself in different ways.
I just checked for updates, and found the latest version is 3.5.7 (I was on 3.5.6), and loaded it, and I can’t replicate the problem.
Maybe they finally fixed it.