Fix 58568: JS strict warnings in mailContextMenus.js. r=timeless sr=alecf

mailnews/base/resources/content/mailContextMenus.js

@@ -323,7 +323,7 @@ function SetMenuItemAccessKey(id, accessKey)
 
 function fillMessagePaneContextMenu(contextMenuNode)
 {
-	contextMenu = new nsContextMenu(contextMenuNode);
+	var contextMenu = new nsContextMenu(contextMenuNode);
 
 	var message = GetLoadedMessage();
 	var numSelected = (message) ? 1 : 0;