On Tuesday, August 30th, Mozilla pushed out emergency updates for both Firefox 3.6 and 6.0. These updates revoke the root certificate for DigiNotar due to fraudulent SSL certificate issuance [Additional Details]. Firefox 3.6 users can update via Help > Check For Updates. Firefox 6.0 users can update via Help > About Firefox. Also updated versions of Firefox 3.6 and 6.0 can be downloaded:
Note: Since Firefox 3.6 uses the old release process, all features, bug fixes, improvements, etc.planned for the upcoming Firefox 3.6.21 release on September 27th are being rolled over to Firefox 3.6.22.
Source: Mozilla Developers News