Fix the MOW height calculation in Muttator.

common/content/configbase.js

@@ -104,6 +104,12 @@ const configbase = { //{{{
      */
     name: null,
 
+    /**
+     * @property {number} The height (px) that is available to the output
+     *     window.
+     */
+    get outputHeight() getBrowser().mPanelContainer.boxObject.height,
+
     /**
      * @property {[string]} A list of extra scripts in the liberator or
      *    application namespaces which should be loaded before liberator