Important Note: One of the malware results has been verified to be a false positive. Further details are available here: http://blog.mozilla.org/addons/2010/02/09/update-on-the-amo-security-issue/
Original blog entry follows below.
Two add-ons in the experimental section of addons.mozilla.org were found to be containing malware. These were not originally detected with the anti-malware scanning tools that we have been using. We have since increased the number of scanning tools, and will be taking additional steps to minimize the risk of further incidents. Full details of the issue and recommended mitigation steps are here on the AMO blog: