Make this work as a Web Extension in Firefox 57

[zarino]

This doesn’t work, out of the box, in Firefox. I think we’re using some Chrome-only APIs. Investigate.

[zarino]

These will come in useful:

• about:debugging
• https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Porting_a_Google_Chrome_extension
• https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Chrome_incompatibilities