Why is Firefox suddenly using all my system resources?

I have Firefox as my default browser, and I always keep it up to date. Recently, it has been using 100% of my processor and up to 280MB of RAM, but doesn’t seem to be doing anything. It has got to the point where I don’t have the remaining speed to even play music while I am online.

This isn’t the first time it has happened, but last time was when I had Firefox 2 (I forget which version). I uninstalled and re-installed back then, and it worked again until now. I don’t want to keep doing this though, so I am looking for a more permanent solution so I don’t have to reconfigure the whole program.

My computer is otherwise healthy, running XP SP3. I have not installed any new plugins for several months, and I cannot identify any specific event that led to this happening. I am now using Chrome until I can get Firefox fixed.