At the moment, add-ons can call and customize plenty of things on the main thread, but nothing on workers. It would be easy to extend PromiseWorker to support: - injecting code; - watching messages sent and received. I believe that we should do this. This will immediately open an extension API for the OS.File Worker, the Session Restore Worker and the Thumbnail Worker, as well as let us test more efficiently stuff that goes on in said workers.