diff --git a/mozilla/xpcom/io/nsILocalFileMac.idl b/mozilla/xpcom/io/nsILocalFileMac.idl
index d1a4b7507bd..e0640bc767d 100644
--- a/mozilla/xpcom/io/nsILocalFileMac.idl
+++ b/mozilla/xpcom/io/nsILocalFileMac.idl
@@ -158,6 +158,13 @@ interface nsILocalFileMac : nsILocalFile
     */
     void openDocWithApp(in nsILocalFile aAppToOpenWith, in boolean aLaunchInBackground);
 
+   /**
+    * isPackage
+    *
+    * returns true if a directory is determined to be a package under Mac OS 9/X
+    *
+    */
+    boolean isPackage();
 };
 
 %{C++
diff --git a/mozilla/xpcom/io/nsLocalFileMac.cpp b/mozilla/xpcom/io/nsLocalFileMac.cpp
index f730ac4abbd..294cfd5411c 100644
--- a/mozilla/xpcom/io/nsLocalFileMac.cpp
+++ b/mozilla/xpcom/io/nsLocalFileMac.cpp
@@ -2059,6 +2059,41 @@ nsLocalFile::Exists(PRBool *_retval)
     return NS_OK;
 }
 
+NS_IMETHODIMP  
+nsLocalFile::IsPackage(PRBool *outIsPackage)
+{
+    NS_ENSURE_ARG(outIsPackage);
+    *outIsPackage = PR_FALSE;
+
+    // Note: IsDirectory() calls ResolveAndStat() & UpdateCachedCatInfo
+    PRBool isDir;
+    nsresult rv = IsDirectory(&isDir);
+    if (NS_FAILED(rv)) return rv;
+
+    *outIsPackage = ((mCachedCatInfo.dirInfo.ioFlAttrib & kioFlAttribDirMask) &&
+                     (mCachedCatInfo.dirInfo.ioDrUsrWds.frFlags & kHasBundle));
+
+    if ((!*outIsPackage) && isDir)
+    {
+        // Believe it or not, folders ending with ".app" are also considered
+        // to be packages, even if the top-level folder doesn't have bundle set
+        nsXPIDLCString name;
+        if (NS_SUCCEEDED(rv = GetLeafName(getter_Copies(name))))
+        {
+            const char *extPtr = strrchr(name, '.');
+            if (extPtr)
+            {
+                if (!nsCRT::strcasecmp(extPtr, ".app"))
+                {
+                    *outIsPackage = PR_TRUE;
+                }
+            }
+        }
+    }
+
+    return NS_OK;
+}
+
 NS_IMETHODIMP  
 nsLocalFile::IsWritable(PRBool *outIsWritable)
 {