Mozilla have pushed out updates to users of Firefox 3 and 3.5. The updates include several security and stability fixes.
Firefox 3.5.6 fixes 3 critical, 1 high, 2 moderate, and 1 low level security vulnerabilities.
Firefox 3.0.16 fixes 1 critical, 1 high, 2 moderate, and 1 low level security vulnerabilities.
A full list of changes can be found in the Firefox 3.5.6 and Firefox 3.0.16 release notes.
The update will be pushed out to existing users, or can be downloaded from the Firefox website.