The next Beta development milestone for Firefox 4.0 is planned for the week of September 6th, 2010.
In fact, Mozilla noted
during the latest Firefox Planning meeting that it is targeting Monday September 6th for the release, but obviously, this deadline is by no means set in stone.
The previous Beta Builds of Firefox 4.0 have suffered small delays on more than one occasion, and of course, there’s always the chance that this could happen again.
But even in a worst case scenario, early adopters will only see Firefox 4.0 Beta 5 postponed by just a few days and nothing more.