Firefox 3.5.9 was released on March 30th. 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. Also, a reminder from Mozilla Developer Network, 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 3.5.9 from www.firefox.com.