Mozilla has temporarily stopped delivering the latest version of Firefox 16.0, due to a security vulnerability.
Version 15.0.1 is considered "up to date" while Mozilla resolves the problem.
The vulnerability, which potentially allows an attacker to view a user's browsing history, only affects version 16; Firefox 15 is unaffected.
"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," writes Mozilla's Director of Security Assurance Michael Coates.
Mozilla will automatically upgrade users' browser to the new version once the problem is fixed. Users who had already upgraded to version 16 can downgrade back to Firefox 15.0.1 by following these instructions.