Two experimental add-ons, Sothink Web Video Downloader and all versions of Master Filer contain Trojan code aimed at Windows users. Uninstalling these extensions does not remove the trojan from a users system. Users with these extensions should uninstall them immediately. The trojan would be executed when Firefox starts and the host computer would be infected by the Trojan. This vulnerability is known to affect Firefox on Windows only, if either Master Filler or Version 4.0 are installed. No other instances of malware have been discovered.”]
Source: https://blog.mozilla.org/addons/2010/02/04/please-read-security-issue-on-amo/