Refactor and upstream patch for #10280
Mozilla's team reviewed and discussed our patch for #10280 (moved). https://bugzilla.mozilla.org/show_bug.cgi?id=817255#c20
First, after new GMP (https://wiki.mozilla.org/GeckoMediaPlugins), patch with ESR-38 could be wrong/invalid/incomplete/broken.
Second, they don't want new UI elements for about:addons (Plugins). Do we need this UI elements? Isn't Torbutton's menu (Security settings) should be enough to allow or disallow plugins? Our FAQ https://www.torproject.org/docs/faq.html.en#TBBFlash says nothing about steps to allow Flash neither?
In general, we could to simplify patch a lot using all comments from review, or discuss about race conditions that simplify could lead to.