While it is working on a fix, Mozilla has removed the download of Firefox 16 and replaced it with Firefox 15.0.1, which is not affected by the issue. Users who already upgraded to version 16 are recommended to downgrade back to version 15.
According to Mozilla, the confirmed vulnerability "could allow a malicious site to potentially determine which websites users have visited and have access to the URL or URL parameters." Mozilla said it is not aware of any public exploits at this time.
Users who do not want to downgrade to version 15, "can wait until [Mozilla's] patches are issued and automatically applied to address the vulnerability," wrote Mozilla's Michael Coates in a blog post. Of course, that would also mean that the security issue will persist until the fix is available.
[UPDATE] Panic over, everyone. Reader Ira Milner tells us that Firefox 16.01 is now available for download. You can grab it here.