General :: Register A Custom Protocol With Xdg?

I've been struggling this morning trying to associate an application with a custom protocol, namely emacsclient and org-protocol.

I'm calling this protocol from a webbrowser bookmarklet, and I get the following behaviour :

In chromium, the "Launch Application" dialog comes up, and calls xdg-open org-protocol://... which ends up firing a new chromium frame.

In firefox, I've tried setting network.protocol-handler.app.org-protocol to an empty string or my emacsclient path, anyhow I get the following error message : "Firefox doesn't know how to open this address, because the protocol (org-protocol) isn't associated with any program" without even showing any external application selection dialog.

I'm not using any desktop environment, so I need to make this work strictly with xdg, however, despite reading the shared mime info spec etc, I still can't fathom a working configuration.