Check for compatibility with multiprocess Firefox

[daveschaefer]

Upcoming versions of Firefox will run code in a different process from content. This may affect extensions - https://developer.mozilla.org/en-US/Add-ons/Working_with_multiprocess_Firefox

We should investigate whether Perspectives is affected and see if we'll need to fix anything.

[daveschaefer]

I ran through the auto-checker at https://compatibility-lookup.services.mozilla.com/ and they believe that yes, Perspectives will be affected because of our use of XUL

https://wiki.mozilla.org/Add-ons/developer/communication#4.29_Has_XUL_or_XPCOM_add-on

We'll have to look into this then.

[daveschaefer]

Perhaps another reason to separate out the browser-specific functionality in #75

[blerner]

Bumping this: https://blog.mozilla.org/addons/2017/02/16/the-road-to-firefox-57-compatibility-milestones/ implies that by November, Perspectives will no longer be compatible with Firefox unless it is converted to a WebExtension. I'm happy to help test beta versions; Perspectives is one of the few extensions remaining on my must-have list that will be incompatible, and I'd certainly be willing to help remedy that!

[myano]

I am happy to report that if you force e10s in Firefox 51/52, Perspectives will continue work (for the time being).