Mozilla on Friday said that it had removed two FIREFOX add-ons from its Web site because they installed malware.

"Two add-ons in the experimental section of addons.mozilla.org were found to be containing malware," Mozilla said on its security blog. "These were not originally DETECTED with the anti-malware scanning tools that we have been using. We have since increased the number of scanning tools, and will be taking ADDITIONAL steps to minimize the risk of further incidents."

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The malicious add-ons have been identified as version 4.0 of Sothink Web Video Downloader and all versions of Master Filer. According to AMO's blog post, Sothink Web Video Downloader 4.0 included malware known as Win32.LdPinch.gen, while Master Filer included malware known as Win32.Bifrose.32.Bifrose Trojan.

Last week, we disclosed two instances of suspected malware in experimental add-ons on AMO. Since that DISCLOSURE, we’ve worked with security experts and add-on developers to determine that the suspected trojan in Version 4.0 of Sothink Video Downloader was a false positive and the extension does not include malware. The same investigation also confirmed that the Master Filer extension included a valid instance of a trojan. Our estimate of 6,000 affected downloads has been revised to under 700. The Sothink Video Downloader has been re-enabled on AMO. We apologize to our users and the developers of Sothink for any inconvenience this has caused.

Thanks to the team at McAfee for working with us to better understand this threat.