The Mozilla team has decided that the beta for Firefox 3.0 will not be ready as early as they wished, following the alpha 5 release a month ago and alpha 6 just a few days ago. The release of the beta has been extended six weeks, into mid September.

Due to the large amounts of changes and revamping found in 3.0, delays like this aren't unexpected at all. They are skeptical to get peoples hopes up by labeling it “beta” when it hasn't left the initial production stage:

”Among the changes: a postponement of the first beta, slated as recently as last week to debut July 31. It's not expected until Sept. 18 at the earliest.
"Based on this criteria, it does not appear that M7 will be ready to be called a beta," Schroepfer wrote.
The alphas of course are still available and they still have releases on the calendar. The final release date has yet to be announced, but if you are a tinkerer the alpha is definitely worth a look.