...verifier from contacting AMO even though we have AMO disabled

from original commit:

With https://bugzilla.mozilla.org/show_bug.cgi?id=1245571 support for
websites to learn about installed add-ons landed. Currently, this is
only enabled for AMO related sites but we don't think this functionality
is something we want for Tor Browser as it might aid in fingerprinting
users.

The patch does not outright disable access to the API. Privileged code
is still able to use it if needed. This should help with usability
issues should they arise while mitigating possible fingerprinting and
security problems by having this API available to content.

This fixes bug 21684 by deleting the whitelist of URLs shipped with
Firefox 52.