Mozilla recently 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."

AMO, Mozilla's add-on management group, recently posted a notice about the malicious add-ons.

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.

Launching Firefox with either of these add-ons installed on a Windows computer is likely to lead to an infection. Removing the add-on does not remove the trojan software, however. Antivirus software that recognizes the malware is necessary for removal. According to Mozilla, the following antivirus apps will work: Antiy-AVL, Avast, AVG, GData, Ikarus, K7AntiVirus, McAfee, Norman, and VBA32.

Last May, security researcher Duarte Silva created a proof-of-concept malicious add-on, or "maladon," to highlight problems in Firefox's add-on security model.

Mozilla has made some security improvements since then, such as locking down Firefox's components directory. But the discovery of infected add-ons on Mozilla's AMO site suggests that additional action is necessary.

A Mozilla spokesperson wasn't immediately available for comment.

Master Filer was downloaded approximately 600 times between September 2009 and January 2010. Version 4.0 of Sothink Web Video Downloader was downloaded approximately 4,000 times between February 2008 and May 2008. AMO's blog post says that versions of Sothink greater than 4.0 are not infected. The latest version, 5.7, is not available through AMO's site, but can be found at Sothink's Web site.

In July, Mozilla launched a program to help add-on developers solicit contributions for the add-ons they post on the AMO site.