navigator.registerProtocolHandler('mailto','http://mail.live.com/secure/start?action=compose&to=%s','Live Mail');

var nsiwchr = Cc["@mozilla.org/embeddor.implemented/web-content-handler-registrar;1"].getService(Ci.nsIWebContentHandlerRegistrar);

nsiwchr.registerProtocolHandler('mailto','http://mail.live.com/secure/start?action=compose&to=%s','Live Mail', null);

var uri = this._checkAndGetURI(aURIString, aContentWindow);

aContentWindow.location.hostname != uri.host)

var fakeContentWindow = {
  document: {
    baseURIObject: {
      asciiHost:"mail.live.com",
      asciiSpec:"http://mail.live.com/secure",
      hasRef:true,
      host:"mail.live.com",
      hostPort:"mail.live.com",
      originCharset:"UTF-8",
      password:"",
      path:"/secure",
      port:-1,
      prePath:"http://mail.live.com",
      ref:"", //369
      scheme:"http",
      spec:"http://mail.live.com/secure",
      specIgnoringRef:"http://mail.live.com",
      userPass:"",
      username:""
    }
  },
  location: {
    hash:"", //#369
    host:"mail.live.com",
    hostname:"mail.live.com",
    href:"http://mail.live.com/secure",
    origin:"http://mxr.mozilla.org",
    pathname:"/secure",
    port:"",
    protocol:"http:",
    search:""
  }
};

nsiwchr.registerProtocolHandler('mailto','http://mail.live.com/secure/start?action=compose&to=%s','Live Mail', fakeContentWindow);

"[object XPCWrappedNative_NoHelper]" scratchpad.js:999
SP_writeAsErrorComment/<() scratchpad.js:999
Handler.prototype.process() Promise-backend.js:863
this.PromiseWalker.walkerLoop() Promise-backend.js:742

var protocolScheme = "mailtoorsomething";
var uri = Services.io.newURI("someuri?with_%s_replacement", null, null);
var name = "Some Name";
var desc = "Some description";

var protocolHandler = Services.io.getProtocolHandler(protocolScheme);
if (!(protocolHandler instanceof Ci.nsIExternalProtocolHandler)) {
  throw new Error("Cannot register handler for built-in protocol");
}

var eps = Cc["@mozilla.org/uriloader/external-protocol-service;1"].
          getService(Ci.nsIExternalProtocolService);
var handlerInfo = eps.getProtocolHandlerInfo(protocolScheme);
var handlers =  handlerInfo.possibleApplicationHandlers;
for (let i = 0; i < handlers.length; i++) {
  let h = handlers.queryElementAt(i, Ci.nsIWebHandlerApp);
  if (h.uriTemplate == uri.spec) {
    throw new Error("Already registered");
  }
}

var handler = Cc["@mozilla.org/uriloader/web-handler-app;1"].
              createInstance(Ci.nsIWebHandlerApp);
handler.name = name;
handler.detailedDescription = desc;
handler.uriTemplate = uri.spec;
handlerInfo.possibleApplicationHandlers.appendElement(handler, false);

handlerInfo.alwaysAskBeforeHandling = false;
handlerInfo.preferredApplicationHandler = handler;
handlerInfo.preferredAction = Ci.nsIHandlerInfo.useHelperApp;

var hs = Cc["@mozilla.org/uriloader/handler-service;1"].
          getService(Ci.nsIHandlerService);
hs.store(handlerInfo);