When I exit Firefox, the firefox.exe process continues to run. I cannot re-launch Firefox unless I first stop the firefox.exe process using Task Manager. How do I fix this problem?

I am running WinXP Pro, SP3. I uninstalled and re-installed Firefox and the problem didn't go away. This problem started about 2 months ago. Firefox always worked fine before that. This happens very frequently -- If I have been using Firefox previously, and I closed the program, when I try to re-launch it, about 50% of the time, I need to use Task Manager to stop a persistently running firefox.exe process.