Port to WebExtension #5
Description
Firefox is deprecating it's old extensions, and is moving to WebExtension.
To use WebExtension, use the Native Messaging API and a client-side external program.
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Native_messaging
The external program will need to be OS-specific, so it may be beneficial to use something like Python that works in Linux, Windows, MacOS.