"This add-on contains code that intercepts login data (password and username) submitted to any website, and sends this data to a remote location".
According to Mozilla, "the add-on Mozilla Sniffer has been downloaded approximately 1,800 times since its submission and currently reports 334 active daily users".
Users shouldn’t panic, uninstalling the add-on stops this behavior. Anybody who has installed this add-on should change their passwords as soon as possible.
Moreover, all current users should receive an uninstall notification within a day or so. More details at blog.mozilla.com