Hello,The FSF specifies that the MPL is not GPL compatible and thus isn't a free software license. Why do you consider MPL-licensed addons free?
In any case, Compact Menu 2 is a brilliant addon which is MPL-licensed and I believe should be added to your list. It is very popular and works with both IceCat and Thunderbird. It basically reduces the Menu bar (File, Edit, View, etc) into a single button.
Thanks, Ruscur -- http://gnuzilla.gnu.org
