Add-ons are a great thing – they make your software more useful and much more to your taste.
Firefox has tons of wonderful add-ons, that turn the browser into a mean piece of software for web developers, web designers or power internet users.
Apparently, the popularity of add-ons also made them a target for malware creators. Two add-ons in the experimental section of addons.mozilla.org were found to be containing malware – Version 4.0 of Sothink Web Video Downloader and all versions of Master Filer. Mozilla has since increased the number of scanning tools, and will be taking additional steps to minimize the risk of further incidents.
This vulnerability is known to affect Firefox on Windows only, if either Master Filer or Version 4.0 of Sothink Web Video Downloader are installed. Versions of Sothink Web Video Downloader greater than 4.0 are not infected.
Simply removing the ad-ons won't clear the trojans, so you need to scan your computer with an anti-virus.
Be careful out there…