fix #76626. leaking nsXULDocument and nsXULTemplateBuilder slows down

subscribe dialog the second time, and asserts like mad in debug builds.
r=varada, sr=bienvenu (and waterson)

mailnews/base/resources/content/subscribe.js

@@ -149,6 +149,16 @@ function SetUpTree(forceToServer)
 }
 
 
+function SubscribeOnUnload()
+{
+  try {
+    gSubscribeTree.database.RemoveDataSource(subscribeDS);
+  }
+  catch (ex) {
+    dump("failed to remove the subscribe ds: " + ex + "\n");
+  }
+}
+
 function SubscribeOnLoad()
 {
 	//dump("SubscribeOnLoad()\n");

mailnews/base/resources/content/subscribe.xul

@@ -37,6 +37,7 @@ Rights Reserved.
         id="subscribeWindow"
         persist="width height screenX screenY"
         onload="SubscribeOnLoad()"
+        onunload="SubscribeOnUnload()"
         onclose="Stop()">
         
 <stringbundle id="bundle_subscribe" src="chrome://messenger/locale/subscribe.properties"/>