The biggest difference I've noticed so far is that it doesn't do its famous hog-the-CPU trick on pages such as Google Spreadsheets. Now that is a big relief for me; I absolutely hated that particular bug.
Make no mistake: I didn't change my mind about my first experience with Firefox 3. I still believe the developers should have allowed their users to do one of the following:
- check the add-on compatibility during the installation
- revert to Firefox 2
- run Firefox 2 and Firefox 3 side by side