I've been using Firefox 3 on the Mac for the past month so I thought I'd share some of my experiences when using it. There release candidate download page is here.
The reason I didn't start using it beforehand was because I was waiting for a supported version of the Del.icio.us bookmarks add-on as it has become indespensible to me. You can get this here.
I run a few add-ons to make it look more integrated with Mac OSX:
* Locationbar 2 for improving the recognition of URLs.
* Fission for the progress in the URL field like Safari.
* GrApple Delicious (Blue) theme which looks very good (and integrated on the Mac).
* And don't forget AdBlock Plus whichever platform you are running on!
With all of those add-ons working (you can also force add-ons to work by using the Nightly Tester Tools) I was ready to use Firefox 3. For a Beta (and now the Release Candidate) it is amazingly stable.
The entire program is much more responsive and lighter on the system resources. The integration and look and feel (with the add-ons) is excellent, although I would prefer if spell-checking with edit fields to be on by default (although there is a hack here).
In general it is a large step forward although I am not using the biggest new feature - the bookmarks replacement "Places". As usual though the major feather in Firefox's cap is the add-ons and once you have them you find it impossible to use anything else.
Also, I am using it on Windows Vista at work and it is fast, responsive and looks much better, although I would prefer not to have the blue tinge to the window. I'd recommend upgrading now because once you've tried 3 you can't go back.