I assume you've read this release from ZDNet (5 Feb 2010)
Mozilla Firefox hit by malware add-ons
Mozilla says a pair of malicious Firefox add-ons slipped by its security checks and infected approximately 4,600 Windows computers over the last five months.
The browser add-ons, described my Mozilla as “experimental,” contained a Trojan horse that executed when Firefox started and infected the host computer.
According to a post on the Mozilla add-ons blog, the malicious add-ons were Version 4.0 of Sothink Web Video Downloader and all versions of Master Filer.
The Sothink Web Video Downloader contained Win32.LdPinch.gen, and Master Filer contained the Win32.Bifrose Trojan. Both add-ons have been been disabled but Mozilla said they were active since September 2009.
Uninstalling these add-ons does not remove the trojan from a user’s system. 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 anti-virus program should be used to scan and remove any infections.