Firefox 4, 5, 6 and 7 to be released before the end of 2011
As far as feature sets go, this is what the roadmap looks like: Firefox 5 will absorb the Account Manager and F1 Simple Sharing add-ons to become built-in features. It looks like Windows 7 64-bit will be officially supported with FF5, too.
Firefox 7's feature list is less clear. Electrolysis (splitting everything into separate processes) and changes to XBL support are mentioned, but nothing certain.
Overall, there's a big emphasis on responsiveness and "shining the primary UI until it gleams."
It'll definitely be interesting to see how Mozilla transitions to a new release cycle -- and it'll definitely mean a lot more Firefox-related news, too!
Update: some conscientious eyes have noticed that the roadmap says 'Draft' at the top, so this could all change...