Firefox 16 just hit the market a couple of days ago, and Mozilla has been forced to pull down the latest version due to a vulnerability. This security threat not only caused Mozilla to take down the latest version, but they are actually encouraging users who have already upgraded to download Firefox 15 and install it on their computer until further notice.
Firefox says that this vulnerability “could allow a malicious site to potentially determine which websites users have visited and have access to the URL or URL parameters.” That being said, Firefox is not aware of any cases of this exploit being used in the wild, but now that the security threat has been made public, it would not surprise anyone to see malicious individuals start taking advantage of it.
To downgrade, users need to go to this page and download the old version of Firefox. There is no reason to take the risk with version 16, so users are advised to install the old version until Firefox is able to issue an update for version 16.
Mozilla says it should have a safe version of Firefox 16 out within the next day or two, so users should be able to get the new features of the browser soon enough. For the time being, do not download version 16 and if you already have downloaded it, take a few seconds to revert your browser to the old, safe version.
As of this writing, Firefox has already issued a safe version of its Android browser, so the desktop version should not be far behind.