Commercial extension

I would like to learn about the licensing considerations of a commercial extension of Thunderbird. The idea is to add a commercial (not open source) DLL (or shared object) to XUL, plus extend XUL such a way that it calls DLL functions at a certain point of the execution. The XUL extension could be an MPL, i.e. available to everyone. While the DLL is not. Does MPL (Mozilla Public License) allow to commercialize Thunderbird this way?

Can somebody answer my question?