Hey chouser,

Have you tried starting Firefox in Safe Mode? Hold down "shift" while Firefox is starting up. If the crash doesn't happen in Safe Mode, you should read Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

There is also a lot of useful stuff in the Troubleshoot Firefox crashes (closing or quitting unexpectedly) Knowledge Base article. If you've recently upgraded, you might want to check out this article as well.

Hopefully this helps!

As of Firefox 4.0 and newer one needs both OSX 10.5+ and Intel hardware.

Firefox 3.6.* from Mozilla is the last to work on the old OSX 10.4.11. http://www.mozilla.org/en-US/firefox/all-older.html (Firefox 3.6.25 was released recently at same time as 9.0).