Different licences (GPL vs MIT)

[sblondon]

The licence file says it's a

GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

However, the licence claimed on the Firefox plugin page is MIT: https://addons.mozilla.org/en-US/firefox/addon/owl/

Which one is correct? I can provide a PR to change the LICENCE file but perhaps the fix should be the page hosted at mozilla.org?