The release of Firefox 4 has been delayed until early 2011.
According to Firefox director Mike Beltzner's message to the mozilla.dev.planning mailing list, "Development on Firefox 4 has not slowed down and strong progress is being made daily. However, based on the delays in completing the "feature complete" Beta 7 milestone against which our Add-on developers and third-party software developers can develop, as well as considering the amount of work remaining to prepare Firefox 4 for final release, we have revised our beta and release candidate schedule."
Firefox 4 had been slated for release in fall 2010. You can see the revised timetable here.