The latest version of Mozilla's Firefox browser has been taken offline after a security vulnerability was discovered.
Users who had upgraded to version 16 were advised to downgrade to the previous safe release until Firefox developers released a fix.
The vulnerability allowed "a malicious site to potentially determine which websites users have visited", Mozilla said.
Update: Mozilla re-released Firefox 16 today after pulling the browser from distribution Wednesday when one of its developers found a critical bug that could be used by attackers to hijack machines. At around 2:30 p.m. ET, Mozilla turned on its upgrade servers and started pushing Firefox 16.0.1 to users who had earlier downloaded the flawed browser, or who were still running version 15 and earlier.