What's New in Firefox 1.5.0.2
Firefox 1.5.0.2 provides native support for Macintosh with Intel Core processors, and stability and security enhancements that are part of our ongoing program to provide a safer Internet experience for our users. We recommend that all Firefox users upgrade to this latest version.

Here's what's new in Firefox 1.5.0.2:

Universal Binary support for Mac OS X which provides native support for Macintosh with Intel Core processors. Firefox supports the enhancements to performance introduced by the new MacIntel chipsets.
Improvements to product stability.
Several security fixes.
The Burning Edge has more detailed lists of notable bug fixes.

Release Date: April 13, 2006

http://www.mozilla.com/firefox/releases/1.5.0.2.html

I updated to the new version a few days ago and it was a nightmare. Very slow with pages hanging. I tried everything, emptying my cache, deleting cookies, running virus scans etc. Initially I was blaming the windows updates but when I used IE as a test, that was running very fast so I knew it was FF. I have just done a system restore to before the update and now everything's back to normal.