I’m currently handling the mailto protocol with a fake URL. It’s just a .local address that is detected by a webRequest listener and redirected based on the user’s settings.
The problem with this is that the Firefox settings UI doesn’t load a favicon, obviously because there is no favicon to load. I also had this issue when I was simply using a local HTML file to handle the mailto request, despite it having a favicon specified.
Is there a way to manually set a custom icon for the protocol handler?