https://www.gnu.org/licenses/license-list.html#MPL
According to this page, MPL 1.1 is not GPL compatible because it has "some complex restrictions." This also makes it non-FSF approved, so I am not sure if it should be considered morally right to promote it on Trisquel. Some of the add-ons issued in the Abrowser Add-on list are licensed under MPL 1.1, such as PDF Downloader. However, I'm not sure whether this was on accident or not as I'm not exactly sure what qualifies for the Add-on list.
