Deprecated since Gecko 2 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1)
This feature has been removed from the Web. Though some browsers may still support it, it is in the process of being dropped. Do not use it in old or new projects. Pages or Web apps using it may break at any time.
The popup element is equivalent to the menupopup element which should be used instead. See the documentation on the menupopup element for more information.