Mozilla published Firefox 16 on Tuesday but at present the update is temporarily removed due to a critical security flaw that the company is fixing asap. A patched version of Firefox 16 is expected to become available later this day, but as a precaution, Mozilla advises users to downgrade to Firefox 15.0.1 to avoid any risk of getting infected by a potential exploit.
Mozilla is aware of a security vulnerability in the current release version of Firefox (version 16). We are actively working on a fix and plan to ship updates tomorrow. Firefox version 15 is unaffected.
The vulnerability could allow a malicious site to potentially determine which websites users have visited and have access to the URL or URL parameters. At this time we have no indication that this vulnerability is currently being exploited in the wild.
Firefox 16 has been temporarily removed from the current installer page and users will automatically be upgraded to the new version as soon as it becomes available. As a precaution, users can downgrade to version 15.0.1 by following these instructions [http://www.mozilla.org/firefox/new/]. Alternatively, users can wait until our patches are issued and automatically applied to address the vulnerability.