The official Mozilla Add-ons Blog has issued a security notice to Firefox users regarding two experimental add-ons that made their way onto the Firefox Add-ons pages containing malware. The extensions in question — Sothink Web Video Downloader and Master Filer (neither of which had we even heard of) — target Windows users and have been removed.
Mozilla reassures users:
AMO performs a malware check on all add-ons uploaded to the site, and blocks add-ons that are detected as such. This scanning tool failed to detect the Trojan in Master Filer. Two additional malware detection tools have been added to the validation chain and all add-ons were rescanned, which revealed the additional Trojan in Version 4.0 of Sothink Web Video Downloader. No other instances of malware have been discovered.