From the how many Beta's makes a release file...

Mozilla is currently in the final stages of making their next generation Firefox 3 browser release. Beta 3 of Firefox 3 should be out in a week or so following testing which begins on Monday February 4th.

Even though full scale testing hasn't yet started, Mozilla developers already know they'll need at least one more Beta - Beta 4 - to iron out a long list outstanding bugs.

According to Mozilla Developer Mike Beltzner , Mozilla has  fixed over one thousand bugs between Firefox 3 Beta 2 and the most recent nightly build. 

In a Mozilla developer newsgroup posting Beltzner, notes that it's not yet known how long a Beta 4 will take to get out the door.

Our goal is to do a quick turnaround on Firefox 3 Beta 4, but we cannot
provide a good estimate until we know the size and scope of blockers
remaining after the Beta 3 codefreeze.

Before going into Beta, Mozilla went through no less than nine Alpha development releases of Firefox 3.