If you are using Greasemonkey, a popular Firefox extension which allows near-omnipotent control of most websites, uninstall it as soon as possible! A major security exploit has been discovered. It is highly recommended that you uninstall Greasemonkey and patiently wait for an updated version to correct this issue.

Update: This security exploit has been resolved by the recently released Greasemonkey v0.3.5. It is recommended that you either install Greasemonkey v0.3.5, or uninstall Greasemonkey immediately.

(hat-tip: Podz)