Update to Firefox 63 extension APIs

[gyng]

Also consider holding off until next Firefox ESR, Chrome support

• Multi-tab select (https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs)
  • Highlight tabs downloaded
• Enable access keys for Firefox (https://bugzilla.mozilla.org/show_bug.cgi?id=1320462)
• Investigate alternative modifier keys for shortcuts (https://bugzilla.mozilla.org/show_bug.cgi?id=1364784)
• menus.getTargetElement might be able to allow us to grab information about an element via content script (element title and more)

[almet]

Yes please!

[gyng]

Multi-tab support, access keys for FF >= 63 enabled in v3.3.0. Modifier keys do not seem to need any changes, and menus.getTargetElement is tracked by #65.