The official release of Firefox 3.6, now in its fifth beta, will be delayed until some time in the first quarter of next year according to a page on their development wiki.
The browser had been schedule for release before the end of the year.
Meanwhile, Mozilla intends to release 3.5.6 and 3.5.8 in January and February, respectively.
As for version 4 of the popular browser? That's now expected late 2010 or early 2011. It had been planned for release Q3 or Q4 of 2010.