Mozilla released Firefox 16 just a couple of days ago, but pulled the download from its website on October 10 after a serious security vulnerability was discovered the day after launch. Mozilla's director of security assurance announced the removal on the Mozilla Security Blog and explained the nature of the issue.
What the Vulnerability Does
The bug has the potential to allow a malicious website to determine which websites a user has previously visited, and to gain access to the URL or URL parameters of those visits. Mozilla confirmed that Firefox 15 is not affected by the vulnerability, so users who never updated to version 16 are not at risk.
What You Should Do
If you updated to Firefox 16, Mozilla is advising users to downgrade to version 15.0.1 by following the instructions on Mozilla's website while a patch is being prepared. Alternatively, you can choose to wait for Mozilla to release a fix, though downgrading sooner is the safer option.
What Firefox 16 Offered
Firefox 16, released on October 9, introduced several new features before the issue was discovered. These included default VoiceOver support on Mac OS X and initial web app support for Windows, Mac, and Linux. It followed Firefox 15 which arrived in August with memory leak fixes and support for the Opus video format, and Firefox 14 in July which added secure HTTPS connections for Google searches.