Just a day after releasing Firefox 16, Mozilla pulled the update citing security concerns. Needless to say this was a pretty unusual move. Typically any security vulnerability present in a major release is fixed through point updates. Removing a new release was a drastic move, which indicated that Mozilla reckoned that the vulnerability had a significant chance of being exploited in the wild.
The vulnerability concerned could allow a malicious site to potentially determine which websites users have visited and have access to the URL or URL parameters. The security vulnerability was actually more of a privacy issue that could become a security issue on stupidly coded websites that use GET to transmit confidential information.
Mozilla released a fix for the Android version yesterday, and an updated desktop version was released moments ago. You can download Firefox 16.01 from here, or you can wait for your Firefox installation to automatically download the latest version.