Mozilla have released Firefox 3.5.4 that includes numerous security fixes including six critical updates.

Full details on the security fixes can be found in the Mozilla security advisory:

http://www.mozilla.org/security/known-vuln.../firefox35.html

Details on the new features and installation instructions can be found in the release notes:

http://www.mozilla.com/en-US/firefox/3.5.4/releasenotes/

If you are running the older Firefox 3.0 similiar security updates are available via version 3.0.15

Full details can be found in the Mozilla security advisory:

http://www.mozilla.org/security/known-vuln.../firefox30.html