Firefox Updated to Version 3.5 today. As always it takes a few weeks for add-ons to be updated to work with the new version. So if you are a heavy add-on or theme user then you might want to wait a week or two.
First impressions is that things seems snappier. I have nothing to back this up but my usual websites seem to load faster.
A new feature that will be interesting to watch is the support for HTML5 <video> and <audio> tags. This allows better embedding in sites. The question will be if they get adopted by the web community. The demo on the firefox site was pretty smooth.
Another feature that has been in Safaria and other browsers in a Private Browsing Mode (nicknamed “porn mode” by many). The idea is not record anything about the sites you are visiting (cookies, history, etc.).