According to the wiki, this is what users should expect:
- platform enhancements
- content and chrome in separate process
- binary component versioning
- new extensibility platform (JetPack)
- end user feature enhancements
- browser sync (Weave)
- new UI on all platforms
- speed and responsiveness improvements
- tracemonkey engine improvements
Firefox 4.0 Mock-Up - courtesy, Mozilla
Firefox 4.0 - Tabs on top - image courtesy, Mozilla
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Before all that, however, there will be 3.6, which will include Windows 7 integration, "awesomeformcomplete" and optimizations for session restore and startup. That's due any time now - scheduled for sometime from the end of the month through November.
Then, in the first or second quarter of next year - sometime in February, March or April - 3.7 is scheduled for release. That upgrade is supposed to improve startup time and run plugins separately to improve stability.