Investigate new WebExtensions API requirements for our extensions

A while ago Mozilla announced (https://blog.mozilla.org/addons/2015/08/21/the-future-of-developing-firefox-add-ons/) that the XUL/XPCOM-based extensions will be deprecated and that add-ons built with WebExtensions are the way forward.

We should investigate what this means for our extensions and report back to Mozilla in order to make sure our requirements are taken into account while designing the APIs.

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