Firefox 3.5.12 was released on September 7th. See complete Release Notes for full details.

Auto-updates should be occurring in the next 24-48 hours, otherwise users may get the latest update by going to Help menu and selecting Check For Updates. If you’re still using Firefox 3.0.x, this version is no longer supported and may contain security vulnerabilities. Please upgrade to Firefox 3.5 by downloading Firefox 3.5.12 from here

It also appears there is going to be one more update after all. Firefox 3.5.13 is due out in mid October 2010.