Most users should automatically move to the new version through the update channel.

Mozilla Firefox 9 Released
http://www.mozilla.org/en-US/firefox/9.0/releasenotes/
http://www.us-cert.gov/current/index.html#mozilla_releases_firefox_9_and

Mozilla Firefox 9 Security issues resolved
http://www.mozilla.org/security/known-vulnerabilities/firefox.html#firefox9