added liberator.config.visualbellWindow for generic vb support.

content/liberator.js

@@ -507,7 +507,7 @@ const liberator = (function () //{{{
                 {
                     // flash the visual bell
                     var popup = document.getElementById("liberator-visualbell");
-                    var win = getBrowser().mPanelContainer;
+                    var win = liberator.config.visualbellWindow;
                     var box = document.getBoxObjectFor(win);
 
                     popup.style.cssText = vbs;

content/muttator.js

@@ -43,6 +43,7 @@ liberator.config = {
                GetThreadTree();
     },
     get mainWindowID() { return this.isComposeWindow ? "msgcomposeWindow" : "messengerWindow"; },
+    get visualbellWindow() { return document.getElementById(this.mainWindowID); },
     isComposeWindow: false,
 
     autocommands: [["FolderLoaded", "Triggered after switching folders in Thunderbird"],

content/muttator.xul

@@ -77,6 +77,8 @@ the terms of any one of the MPL, the GPL or the LGPL.
             <!-- other keys are handled inside the event loop in events.js -->
         </keyset>
 
+        <panel id="liberator-visualbell"/>
+
         <statusbar id="status-bar" class="hl-StatusLine">
             <hbox insertafter="statusTextBox" id="liberator-statusline" flex="1" height="10" hidden="false" align="center">
                 <statusbarpanel class="plain" id="liberator-statusline-field-url" readonly="false" flex="1" crop="end"/>

content/vimperator.js

@@ -36,6 +36,8 @@ liberator.config = { //{{{
     defaults: { guioptions: "" },
     guioptions: { m: ["toolbar-menubar"], T: ["nav-bar"], b: ["PersonalToolbar"] },
 
+    get visualbellWindow() { return getBrowser().mPanelContainer; },
+
     autocommands: [["LocationChange", "Triggered when changing tabs or when naviagtion to a new location"],
                    ["PageLoadPre",    "Triggered after a page load is initiated"],
                    ["PageLoad",       "Triggered when a page gets (re)loaded/opened"],