The page you reference is about combining MPL 2.0 code with GPLed code. That is not what we are doing here - we are relicensing lightbeam under the MPL 2.0 alone. Therefore, the licensing header required is the MPL 2.0 licensing header, which is the one attached to all the files in this pull request.

If someone ever takes the Lightbeam code and makes it part of a larger project which is already GPLed, then they would need to consult that page, and perhaps take some of the actions and/or use the licensing text therein. But until that time, that page is not relevant.

This is one of my jobs at Mozilla. Trust me - the licensing info (headers, added files, removals) in this pull request is correct :-)