From the "not far now..." files:

The next big release from Mozilla - Firefox 3.1 is getting a little bit closer to release (just a little).  According to the latest update from Mozilla, a code freeze for Firefox 3.1 is now set for Nov 4-6 with release builds expected to be available on November 11th.

Though it's only officially a point release - I personally see a lot of changes in Firefox 3.1 over Firefox 3.0.x in terms of speed and functionality. Which means there are a lot of things to test and as always the potential for regressions (which makes the testing more fun). I've got Firefox 3.1 Beta 1 running now and it's reasonably stable, though many Add-Ons don't support it yet (but that's typical for this stage of a release cycle).

I don't doubt that we'll see a Beta 3 of Firefox 3.1 (one hasn't officially been announced yet AFAIK), but I do know Mozilla is always about releasing things when they're done (as opposed to being scheduling zealots).