BackgroundPageThumbs should disable HTML5 media in its browser. Bug 759946 is necessary but not sufficient, since BackgroundPageThumbs's browser is remote. There will need to be some way to send "allow media?" to remote docshells. (It may be helpful to see bug 841495 attachment 744963 [details] [diff] [review] for how ContentParent and TabChild were modified to support remote private browsing.)

Oops, bug 759964 I meant, not 759946.

Actually that might not be true. The content script, backgroundPageThumbsContent.js, could probably just set allowMedia on the docshell. The reason PB mode had to be forwarded to the remote process was that it had to be set up right after the docshell was created. I don't know if that's true for allowMedia, too.

Oh, this bug should set allowPlugins = false, too, unless that turns out to be unexpectedly hairy, warranting its own bug.

This is all that's needed.