Such problems are typically related to problematic extensions which cause the firefox.exe process to continue to linger after the application is closed.
I recently got a computer with a Vista Home Premium 6.0 Service Pack 1 operating system. I also have Firefox 3.0.5. What seems to happen on a frequent basis is Firefox will freeze. When I end the program, Firefox is still taking up RAM space (I have 2gb of RAM). I go to task manager and in Procceses click to end the program. Nothing happens and I have to reboot. Any idea on how to fix