In a blog post today, Mozilla said that two malicious Firefox Add-ons - Version 4.0 of Sothink Web Video Downloader and all versions of Master Filer were found to contain Trojan code aimed at Windows users. These Add-ons were however rated as "Expimental Add-ons" and fortunately did not make it to the main Add-ons section of Firefox. These add-ons have been removed now. We have told you previously how malware was found in Ubuntu Screensavers. 

Mozilla reported that Version 4.0 of Sothink Web Video Downloader contained Win32.LdPinch.gen, and Master Filer contained Win32.Bifrose.32.Bifrose Trojan. The trojan would be executed when Firefox starts and the host computer would be infected by the trojan. Uninstalling the add-ons does not remove the trojan and users who have installed these add-ons are advised to scan their computer with an anti-virus program.

Mozilla also reported that Master Filer was downloaded approximately 600 times between September 2009 and January 2010 and Version 4.0 of Sothink Web Video Downloader was downloaded approximately 4,000 times between February 2008 and May 2008.

Both these malwares are detected by most of the antivirus programs available for both free and paid. As the malware is not new and can be detected by antivirus, it is a surprise that it took this much time for the malware to be found.

[via Mozilla Add-ons Blog]