The latest version is 3.5b99 and is the final step on the road to the Release Candidate. In order to get 3.5b99 you need to download and run 3.5b4 from the website here, then select ‘Check for Updates’. Firefox will offer  you the option of upgrading, then restart. Note that you’ll probably lose a couple more of your add-ons after you upgrade.

The release notes say “We expect that the Firefox 3.5 Release Candidate to be issued shortly. At that time you will be automatically upgraded to that version.”

As for my personal impressions… 3.5 is fast, damn fast. And much lower memory footprint than 3.0. I’ve got a couple of negatives as well, but I want to investigate on a clean machine first before I’m sure about those.