Sandbox Tor Browser's content renderer processes more strictly
Since we have enabled Electrolysis (e10s), we confine these processes in exactly the same way as the parent Firefox process. I’m pretty sure they could be confined much more strictly, without impacting UX whatsoever. And while we’re at it, maybe some permissions we currently grant to the parent Firefox process are not needed anymore, as it does less work.
Feature Branch: feature/12679-sandbox-firefox-content-renderers