MFH: r470668 r470672 r470675

gecko: mask as Linux on AMO for compatibility (r470716 followup)

Many extensions on addons.mozilla.org aren't listed or updated due to
platform whitelist abuse. On DragonFly the search is broken because
such a platform isn't recognized.

PR: 226919
Reported by: Graham Perrin, Corpo (Lightning)
Approved by: ports-secteam blanket