https://bugzilla.mozilla.org/show_bug.cgi?id=1255039 and dependent bugs implemented a way in which addons.mozilla.org is able to gather data about extensions the user has installed (and be it just a "yes/no"). We don't want this.
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FWIW: I pondered quite a while whether we should disable this API for both chrome and content but finally opted for doing so just for the latter. There might be breakage involved (especially in the longer run) by not allowing Firefox internals to use it. However, I am not sold to this. Thus, if there are good arguments for kicking window.navigator.AddonManager fully out let me know and we can reconsider it.
Trac: Status: assigned to needs_review Keywords: TorBrowserTeam201705 deleted, TorBrowserTeam201705R added
r=mcs
Looks good to me, and the approach seems sound as well.
My only nit is that in the commit message you should s/Priviledged/Privileged/ (remove the extra 'd').