Mozilla developers are now testing the next stable build of the Firefox 3.5.x series. Firefox 3.5.7 is now available as a beta build for testers to try out.
The 3.5.7 dev update milestone follows closely on the heels of the Firefox 3.5.6 update which came out earlier this month.
Mozilla is not in the habit of disclosing any particular security vulnerabilities that may be patched for upcoming releases (a practice I respect and admire). That said they do publicly disclose the key non-security bug fixes in upcoming releases.