Firefox has a lot of stuff going on under the covers, and each add-on/extension can introduce instabilities and memory leaks. Firefox 2.x had terrible memory leaks for me (on startup with 2-3 dozen tabs its memory usage would be ~ 200 MB, but it would eventually grab a GB of RAM or more if left open for a week or so). Firefox 3.x is much better in that respect, for me anyway.

Otherwise, you might try running MemTest86 for a few hours (it's on most Linux boot disks). IME, flaky RAM causes lockups instead of slowdowns, but running MemTest86 would at least stress the machine with something systematic.

HTH a bit. Good luck!

Cheers,
Scott.