Make NoScript work on the highest security level on protected Mozilla websites

https://bugzilla.mozilla.org/show_bug.cgi?id=1453988 raised awareness for a new mechanism to regulate webextension interference with priviledged Mozilla sites. We need to clear the whitelist that got implemented in https://bugzilla.mozilla.org/show_bug.cgi?id=1415644. https://bugzilla.mozilla.org/show_bug.cgi?id=1453988#c0 gives a good introduction about the trade-offs involved and we might think about a better solution while preparing our switch to ESR60 (or do so in a follow-up bug).

One idea tjr had was to get a patch developed to make an exemption to that whitelist for Mozilla-signed extensions. We could then piggyback on that by signing all the extensions we ship which we thought about doing anyway.