Firefox keeps opening new processes

I've tried searching online a lot for this problem and what may be causing it, but I can't seem to find anyone with the exact same problem.

Basically, Firefox isn't working properly. When I open it, it runs fine, and when I close it it seems to open back up okay. But the problem is, every time I open + close it, a firefox process keeps running in the background. There is nothing stopping me from opening a new firefox instance, but each time I do, it creates a new process, until I have dozens running in the background and my computer slows to a crawl. I have to open up the task manager and kill each process individually to restore performance.

I'm running Firefox on a Windows XP computer, and I tried completely uninstalling firefox (I was running version 2-point-something or other at this point) and re-installing the new version 3-point-whatever, with no success. Any idea what the problem could be?