One day after Firefox 16 was made available, Mozilla has had to pull the browser over security fears. If you upgraded, you might want to roll back until it's fixed.
Mozilla says the software is "temporarily removed", which suggests the flaw is reasonably serious. In a blog post, Mozilla explains:
"The vulnerability could allow a malicious site to potentially determine which websites users have visited and have access to the URL or URL parameters... At this time we have no indication that this vulnerability is currently being exploited in the wild."