diff --git a/mozilla/dom/src/base/nsMimeTypeArray.cpp b/mozilla/dom/src/base/nsMimeTypeArray.cpp
index c676a0ccc04..c10d4f3cabc 100644
--- a/mozilla/dom/src/base/nsMimeTypeArray.cpp
+++ b/mozilla/dom/src/base/nsMimeTypeArray.cpp
@@ -143,17 +143,17 @@ MimeTypeArrayImpl::NamedItem(const nsAString& aName,
       nsMIMEInfoHandleAction action = nsIMIMEInfo::saveToDisk;
       mimeInfo->GetPreferredAction(&action);
       if (action != nsIMIMEInfo::handleInternally) {
-        nsCOMPtr<nsIFile> helper;
-        mimeInfo->GetDefaultApplicationHandler(getter_AddRefs(helper));
-        if (!helper) {
+        PRBool hasHelper = PR_FALSE;
+        mimeInfo->GetHasDefaultHandler(&hasHelper);
+        if (!hasHelper) {
+          nsCOMPtr<nsIFile> helper;
           mimeInfo->GetPreferredApplicationHandler(getter_AddRefs(helper));
           if (!helper) {
-            // mime info from the OS may not have a PreferredApplicaitonHandler
+            // mime info from the OS may not have a PreferredApplicationHandler
             // so just check for an empty default description
             nsXPIDLString defaultDescription;
             mimeInfo->GetDefaultDescription(getter_Copies(defaultDescription));
             if (defaultDescription.IsEmpty()) {
-
               // no support; just leave
               return NS_OK;
             }
diff --git a/mozilla/embedding/components/ui/helperAppDlg/nsHelperAppDlg.js b/mozilla/embedding/components/ui/helperAppDlg/nsHelperAppDlg.js
index 3dbc0abf4fa..169ebe4ed42 100644
--- a/mozilla/embedding/components/ui/helperAppDlg/nsHelperAppDlg.js
+++ b/mozilla/embedding/components/ui/helperAppDlg/nsHelperAppDlg.js
@@ -374,7 +374,7 @@ nsHelperAppDialog.prototype = {
             // On other platforms, default is Ok if there is a default app.
             // Note that nsIMIMEInfo providers need to ensure that this holds true
             // on each platform.
-            result = this.mLauncher.MIMEInfo.defaultApplicationHandler;
+            result = this.mLauncher.MIMEInfo.hasDefaultHandler;
         }
         return result;
     },
diff --git a/mozilla/netwerk/mime/public/nsIMIMEInfo.idl b/mozilla/netwerk/mime/public/nsIMIMEInfo.idl
index e7a205e3f8d..6421151e0a9 100644
--- a/mozilla/netwerk/mime/public/nsIMIMEInfo.idl
+++ b/mozilla/netwerk/mime/public/nsIMIMEInfo.idl
@@ -135,11 +135,17 @@ interface nsIMIMEInfo : nsISupports {
      */
     attribute wstring applicationDescription;
 
+    /**
+     * Indicates whether a default application handler exists,
+     * i.e. whether the defaultApplicationHandler attribute is valid
+     */
+    readonly attribute boolean hasDefaultHandler;
+
     /**
      * Returns a nsIFile that points to the application that is associated
      * by default with this content type.  This will usually be specified in
-     * the platform settings somehow.  This is not always guaranteed
-     * to be set!!
+     * the platform settings somehow.  This is only guaranteed to be set if
+     * hasDefaultHandler is true!
      */
     attribute nsIFile defaultApplicationHandler;
 
diff --git a/mozilla/netwerk/mime/src/nsMIMEInfoImpl.cpp b/mozilla/netwerk/mime/src/nsMIMEInfoImpl.cpp
index 80d90c18a7a..cc67ddce4a9 100644
--- a/mozilla/netwerk/mime/src/nsMIMEInfoImpl.cpp
+++ b/mozilla/netwerk/mime/src/nsMIMEInfoImpl.cpp
@@ -296,6 +296,13 @@ NS_IMETHODIMP nsMIMEInfoImpl::SetPreferredApplicationHandler(nsIFile * aPreferre
   return NS_OK;
 }
 
+NS_IMETHODIMP nsMIMEInfoImpl::GetHasDefaultHandler(PRBool * _retval)
+{
+  *_retval = mDefaultApplication != nsnull;
+  return NS_OK;
+}
+
+
 NS_IMETHODIMP nsMIMEInfoImpl::GetDefaultApplicationHandler(nsIFile ** aDefaultAppHandler)
 {
   *aDefaultAppHandler = mDefaultApplication;
diff --git a/mozilla/uriloader/exthandler/nsExternalHelperAppService.cpp b/mozilla/uriloader/exthandler/nsExternalHelperAppService.cpp
index b861f25cf66..61c475240a8 100644
--- a/mozilla/uriloader/exthandler/nsExternalHelperAppService.cpp
+++ b/mozilla/uriloader/exthandler/nsExternalHelperAppService.cpp
@@ -330,19 +330,20 @@ NS_IMETHODIMP nsExternalHelperAppService::DoContent(const char *aMimeContentType
   else if (url)
   {
     // Get default app/description.
-    nsCOMPtr<nsIFile> defaultApp;
-    nsXPIDLString     defaultDescription;
-    mimeInfo->GetDefaultApplicationHandler(getter_AddRefs(defaultApp));
+    PRBool        hasDefaultApp = PR_FALSE;
+    nsXPIDLString defaultDescription;
+    mimeInfo->GetHasDefaultHandler(&hasDefaultApp);
     mimeInfo->GetDefaultDescription(getter_Copies(defaultDescription));
     // If neither description nor app are specified, then we try to get
     // these from the per-platform OS settings based on the file extension.
-    if (defaultDescription.IsEmpty() && !defaultApp)
+    if (defaultDescription.IsEmpty() && !hasDefaultApp)
     {
       nsCOMPtr<nsIMIMEInfo> osInfo;
       url->GetFileExtension(fileExtension);
       if (NS_SUCCEEDED(GetMIMEInfoForExtensionFromOS(fileExtension.get(), getter_AddRefs(osInfo))))
       {
         // Extract default application and default description.
+        nsCOMPtr<nsIFile> defaultApp;
         osInfo->GetDefaultApplicationHandler(getter_AddRefs(defaultApp));
         osInfo->GetDefaultDescription(getter_Copies(defaultDescription));
         // Copy to result mime info object.
diff --git a/mozilla/uriloader/exthandler/nsMIMEInfoImpl.cpp b/mozilla/uriloader/exthandler/nsMIMEInfoImpl.cpp
index 80d90c18a7a..cc67ddce4a9 100644
--- a/mozilla/uriloader/exthandler/nsMIMEInfoImpl.cpp
+++ b/mozilla/uriloader/exthandler/nsMIMEInfoImpl.cpp
@@ -296,6 +296,13 @@ NS_IMETHODIMP nsMIMEInfoImpl::SetPreferredApplicationHandler(nsIFile * aPreferre
   return NS_OK;
 }
 
+NS_IMETHODIMP nsMIMEInfoImpl::GetHasDefaultHandler(PRBool * _retval)
+{
+  *_retval = mDefaultApplication != nsnull;
+  return NS_OK;
+}
+
+
 NS_IMETHODIMP nsMIMEInfoImpl::GetDefaultApplicationHandler(nsIFile ** aDefaultAppHandler)
 {
   *aDefaultAppHandler = mDefaultApplication;