Refactor as webextension

[bennypowers]

Would be swell to use this in Firefox and pals

There's a guide here https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Chrome_incompatibilities

and here https://blog.yorkxin.org/2017/09/25/things-i-learned-from-migrating-a-chrome-extension-to-firefox-using-webextensions.html