I was working with Firefox since I switched yesterday for version 1.5.
At first it crashed when I visited this link:
Working with Files in PHP, Part 2
I uninstalled all my extensions, and it started working fine. Until I tried to go to the next part of the article.
Working with Files in PHP, Part 3
Since I had uninstalled all my extensions, it seems it might be a bug in Firefox 1.5, or my installation is corrupted somehow.

I have work to do, so back to using Safari for now.

Update: my friend says (in the comments) it works fine on his Windows machine. That could mean its a bug either on the OS X version, or as I said corrupt config on my system. I got to those links via this page.

Update: Dec 5th, I started up firefox just to see if it still crashes. A couple of reboots later, I'm having no problems. Looks like it was a configuration problem, fixed by a couple of reboots.