One of the downsides to Mozilla's
rapid release schedule for Firefox
is that new updates are at risk of being rushed to the public before they're ready for prime time. We're not sure if that's what happened here, but we do know that Mozilla yanked Firefox 16 from the web just a few hours after posting the download link, and it was because of a security issue.
"The vulnerability could allow a malicious site to potentially determine which websites users have visited and have access to the URL or URL parameters," Mozilla explained on its security blog. "At this time we have no indication that this vulnerability is currently being exploited in the wild."
Be that as it may, Mozilla was spooked enough to remove Firefox 16 from its installer page, which has been replaced with version 15.0.1. As a precaution, Mozilla says you can downgrade to the previous version.
"Alternatively, users can wait until our patches are issued and automatically applied to address the vulnerability," Mozilla added.
Whether you intended to upgrade or not, you might still be running Firefox 16. Mozilla recently switched to a silent update scheme that downloads new builds in the background and applies them when you restart your browser. To see which version you're running, click on the Firefox icon in the upper left-hand corner, expand the Help menu, and select About Firefox.