Clicking on mailto: links result in a "Choose application" dialog instead of using GNOME's preferred applications.

xulrunner-1.9/uriloader/exthandler/nsExternalHelperAppService.cpp:

[...]
if (!alwaysAsk && (preferredAction == nsIHandlerInfo::useHelperApp ||
                     preferredAction == nsIHandlerInfo::useSystemDefault))
    return handler->LaunchWithURI(uri, aWindowContext);

  nsCOMPtr<nsIContentDispatchChooser> chooser =
    do_CreateInstance("@mozilla.org/content-dispatch-chooser;1", &rv);
  NS_ENSURE_SUCCESS(rv, rv);

  return chooser->Ask(handler, aWindowContext, uri,
                      nsIContentDispatchChooser::REASON_CANNOT_HANDLE);

alwaysAsk is true because of nsExternalHelperAppService::SetProtocolHandlerDefaults
[...]
nsCAutoString warningPref(kExternalWarningPrefPrefix);
    warningPref += scheme;
    PRBool warn = PR_TRUE;
    nsresult rv = prefs->GetBoolPref(warningPref.get(), &warn);
    if (NS_FAILED(rv)) {
      // no scheme-specific value, check the default
      prefs->GetBoolPref(kExternalWarningDefaultPref, &warn);
    }
    aHandlerInfo->SetAlwaysAskBeforeHandling(warn);

warningPref is network.protocol-handler.warn-external.PROTO and should be false.

Ok to commit?