According to Mozilla, beta 4 will be the last beta before the final version of Firefox 3.5 is released.
As stated by Mike Beltzner, Mozilla’s Director of Firefox, all of the remaining beta issues in Firefox 3.5 have been worked out. And, while they don’t rule out the possibility of beta 4 uncovering additional issues, they fully believe that they’re on track for a release sometime in late Q2.
Mozilla is estimating that nearly 900,000 people are currently using the beta versions of Firefox 3.1/3.5, and they hope that all of this support will allow them to release full versions more regularly. While it took them about two years to release 3.0, it’s only taken them one to develop 3.5.