Since it already had to push Firefox 4.0 into 2011, Mozilla does not want more delays for the new browser and, as stated by Damon Sicore, it is pushing hard so as to finish the software before the end of February.
According to Sicore, who's Sr. Director of Engineering at Mozilla, there are currently 160 hard blockers (serious bugs that need to be addressed before shipping) left so the Firefox team needs to keep the pedal to the metal in the next six weeks to have everything working as intended.
Another beta is in plan to test the latest fixes, while a first RC (Release Candidate) build should be available early next month. All in all, Firefox 4.0 is looking good for Q1 so get ready to update. Unless you're using Chrome.