Mozilla has confirmed a security threat from 2 malicious Firefox add-ons.
The following is quoted from the Mozilla Add-ons Blog in the link below.
["Two experimental 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. Version 4.0 of Sothink Web Video Downloader contained Win32.LdPinch.gen, and Master Filer contained Win32.Bifrose.32.Bifrose Trojan. Both add-ons have been disabled on AMO.
Users with either of these add-ons should uninstall them immediately. Since uninstalling these extensions does not remove the trojan from a user’s system, an antivirus program should be used to scan and remove any infections."]