diff --git a/js/src/xpconnect/loader/mozJSSubScriptLoader.cpp b/js/src/xpconnect/loader/mozJSSubScriptLoader.cpp
index 10ccca21fb33..765b7b493064 100644
--- a/js/src/xpconnect/loader/mozJSSubScriptLoader.cpp
+++ b/js/src/xpconnect/loader/mozJSSubScriptLoader.cpp
@@ -323,11 +323,18 @@ mozJSSubScriptLoader::LoadSubScript (const PRUnichar * aURL
         tmp.Append(uriStr);
 
         uriStr = tmp;
-    }        
-        
-    rv = NS_OpenURI(getter_AddRefs(instream), uri, serv,
-                    nsnull, nsnull, nsIRequest::LOAD_NORMAL,
-                    getter_AddRefs(chan));
+    }
+
+    // Instead of calling NS_OpenURI, we create the channel ourselves and call
+    // SetContentType, to avoid expensive MIME type lookups (bug 632490).
+    rv = NS_NewChannel(getter_AddRefs(chan), uri, serv,
+                       nsnull, nsnull, nsIRequest::LOAD_NORMAL);
+    if (NS_SUCCEEDED(rv))
+    {
+        chan->SetContentType(NS_LITERAL_CSTRING("application/javascript"));
+        rv = chan->Open(getter_AddRefs(instream));
+    }
+
     if (NS_FAILED(rv))
     {
         errmsg = JS_NewStringCopyZ (cx, LOAD_ERROR_NOSTREAM);