Firefox Release Candidate 1 is available for download and I decided to give it a go. My first impressions are quite positive - performance is must better on the same machine, the design has been improved and they have added quite a few usability features.
For instance, when you log into a username/password protected web site, Firefox will propose you to save your credentials only after you've logged in successfully, while previous versions would ask you to save them every time before the submission. Another nice feature is much more visible site security information, especially for HTTPS-enabled web site.
Most of my favourite extensions worked out of the box, but a few were not compatible, most notably Tab Mix Plus and Dictionary Switcher. Fortunately, for the first one I could find a development build fixing the problems, but the second is still broken.