add system URL scheme to open in Tor Browser
There is at least one scenario where it would be useful for other applications to be able to open an URL in Tor Browser. Facebook has special handling for .onion URLs and already prompts users who click on such a link that it is a Tor link and and may not work (if they are not connected to Facebook via the Facebook .onion and they are not coming from a known Tor exit node). If we changed Tor Browser to register a custom URL scheme such as tor, then Facebook and others could offer to open the .onion URL in Tor Browser.
If we do this, it would involve registering an URL scheme with the OS (which would allow other applications to detect that Tor Browser is installed).