Main changelogs:
http://www.mozilla.org/en-US/firefox/21.0/releasenotes/
Complete list of bugs fixed (681 bugs fixed)
Security changelogs
https://www.mozilla.org/security/known-vulnerabilities/firefox.html
Main changelogs:
http://www.mozilla.org/en-US/firefox/21.0/releasenotes/
Complete list of bugs fixed (681 bugs fixed)
Security changelogs
https://www.mozilla.org/security/known-vulnerabilities/firefox.html