An update to Firefox for Windows, Mac and Linux was released at 12pm PT on Oct 11. Users will be automatically updated and new downloads via http://www.mozilla.org/firefox/new/ will receive the updated version (16.0.1).
A fix for the Android version of Firefox was released at 9pm PT on Oct 10.
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.