That’s plain wrong ? Last time I checked you can only do that on developer edition of Firefox otherwise you can only install it as a « temporary extension » that remove itself on next restart. Unless I’m missing something ?
And I would like to add that one can download "Yattee" (Privacy oriented video player for iOS, tvOS and macOS), and add any instance from this list by adding its URL in Parameters > Instances > Personalized instance. It's really an awesome client.
well okay, indeed I was wrong it seems tied to signing, however I thought Mozilla revoked that signature once it removed it from their store.