Check WebExtensions API compability

[RoelN]

Since Safari joined the other browsers: https://hacks.mozilla.org/2020/06/welcoming-safari-to-the-webextensions-community/

and

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Browser_support_for_JavaScript_APIs

In this issue I want to:

• Check for incompatible code
• Rewrite to be compatible

But not:

• Release and test extension for "all" browsers