After a few annoying Firefox crashes I figured out that Shockwave Flash was freaking out on me.
How I Figured It Out
I figured out it was Shockwave Flash plugin by visiting seveal gaming sites and watching the cursor spin and spin. I had to Force quit the browser on my 10.6 by going to Mac->Force Quit->Firefox. I restarted Firefox and disabled all the add-ons by Firefox->Tools-Add-Ons. Went back to a gaming site. The site asked me to install Flash. I then re-enabled Shockwave Flash 10.1.x and watch the browser crash.