Mozilla announced Firefox 4 will not be able to meet its November launch date, instead the browser has been further delayed to early 2011. Firefox director Mike Beltzner explained on mozilla.dev.planning that work on Firefox 4 is taking longer than expected as the company tracks down regressions and sources of instability:
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: https://wiki.mozilla.org/Firefox/4/Beta
The frequent beta releases have been extremely helpful in identifying compatibility issues with existing web content, so we plan on continuing to release beta milestones through the end of December. Our estimate is now that release candidate builds will ship in early 2011, with a final release date close behind. Please note that, as always, this schedule is subject to change based on feedback from users and community members.