On this extension page it claims it is Mozilla Public License 2.0, but on the linked Github site, it says it is MIT license. Further, that repo contains only the README, license, privacy policy and a few image files. There is no code there, and seems there never has been.

AMO add-ons do not have to be OSS of course, but I believe not providing the source is a license violation? The whole thing is shady.

This extension might be legit, but at the least the author should correct the license and provide the source code.