--- a/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -691,20 +691,23 @@ nsresult nsExternalHelperAppService::Get
 NS_IMETHODIMP nsExternalHelperAppService::ExternalProtocolHandlerExists(const char * aProtocolScheme,
                                                                         PRBool * aHandlerExists)
 {
   nsCOMPtr<nsIHandlerInfo> handlerInfo;
   nsresult rv = GetProtocolHandlerInfo(
       nsDependentCString(aProtocolScheme), getter_AddRefs(handlerInfo));
   NS_ENSURE_SUCCESS(rv, rv);
 
-  // See if we have a preferred handler app for this
-  nsCOMPtr<nsIHandlerApp> preferredApp;
-  handlerInfo->GetPreferredApplicationHandler(getter_AddRefs(preferredApp));
-  if (preferredApp) {
+  // See if we have any known possible handler apps for this
+  nsCOMPtr<nsIMutableArray> possibleHandlers;
+  handlerInfo->GetPossibleApplicationHandlers(getter_AddRefs(possibleHandlers));
+
+  PRUint32 length;
+  possibleHandlers->GetLength(&length);
+  if (length) {
     *aHandlerExists = PR_TRUE;
     return NS_OK;
   }
 
   // if not, fall back on an os-based handler
   return OSProtocolHandlerExists(aProtocolScheme, aHandlerExists);
 }
 

--- a/uriloader/exthandler/nsIExternalProtocolService.idl
+++ b/uriloader/exthandler/nsIExternalProtocolService.idl
@@ -54,19 +54,24 @@ interface nsIHandlerInfo;
  * You can ask the external protocol service if it has an external
  * handler for a given protocol scheme. And you can ask it to load 
  * the url using the default handler.
  */
 [scriptable, uuid(7968ffa9-bb51-4b2d-aad2-8ea46c15d27f)]
 interface nsIExternalProtocolService : nsISupports
 {
   /**
-   * Check whether a handler for a specific protocol exists.
+   * Check whether a handler for a specific protocol exists.  Specifically,
+   * this looks to see whether there are any known possible application handlers
+   * in either the nsIHandlerService datastore or registered with the OS.
+   *
    * @param aProtocolScheme The scheme from a url: http, ftp, mailto, etc.
+   *
    * @return true if we have a handler and false otherwise.
+   *
    * XXX shouldn't aProtocolScheme be an ACString like nsIURI::scheme?
    */
   boolean externalProtocolHandlerExists(in string aProtocolScheme);
 
   /**
    * Check whether a handler for a specific protocol is "exposed" as a visible
    * feature of the current application.
    *