fx_cast

The closest I've seen is this extension: https://github.com/hensm/fx_cast If the answer is proprietary red tape from Google, is there a front runner for a casting protocol or dongle we can use?

That is not misleading at all. Many add-ons require such companion apps. Any add-on that wants to access the filesystem in a more broad way or needs to use raw sockets will need this type of native app to be present. There is a protocol for talking between the native app and the WebExtension called "Native Messaging": https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web... Native Messaging allow an Add-on to launch a native application that follows its rules. Basically, the add-on needs to ask for nativeMessaging permission, as this one does in: https://github.com/hensm/fx_cast/blob/master/ext/src/manifes... And the native application needs to whitelist the add-on as someone who can call them, as seen in: https://github.com/hensm/fx_cast/blob/master/app/bin/build.j... You can see similar setups in many of the WebExtensions that ar