Don't use a popuppanel for the bell on Gecko<2 either.

common/content/configbase.js

@@ -263,7 +263,7 @@ const ConfigBase = Class(ModuleBase, {
             right:    0     !important;
         }
 
-        !Bell         border: none; background-color: black;
+        !Bell         background-color: black !important;
         Hint;;* {
             /* This gets released into the wild, so everything is important */
             font: bold 10px monospace !important;

common/content/dactyl.js

@@ -146,16 +146,8 @@ const Dactyl = Module("dactyl", {
                 util.timeout(function () { popup.parentNode.selectedPanel = restore; }, 20);
             }
             else {
-                popup = document.getElementById("dactyl-popup-bell");
-                let win = config.visualbellWindow;
-                let rect = win.getBoundingClientRect();
-                let width = rect.right - rect.left;
-                let height = rect.bottom - rect.top;
-
-                // NOTE: this doesn't seem to work in FF3 with full box dimensions
-                popup.openPopup(win, "overlap", 1, 1, false, false);
-                popup.sizeTo(width - 2, height - 2);
-                util.timeout(function () { popup.hidePopup(); }, 20);
+                highlight.highlightNode(document.documentElement, "Bell");
+                util.timeout(function () { document.documentElement.removeAttributeNS(NS, "highlight"); }, 20);
             }
         }
         else {

common/content/dactyl.xul

@@ -28,9 +28,6 @@
     <window id="&dactyl.mainWindow;">
 
         <popupset>
-            <panel id="dactyl-popup-bell" dactyl:highlight="Bell"
-                   noautohide="true" noautofocus="true"/>
-
             <menupopup id="dactyl-contextmenu"
                        onpopupshowing="return (event.target != this || dactyl.modules.commandline.onContext(event));">
                 <menuitem id="dactyl-context-copylink"

common/skin/dactyl.css

@@ -70,7 +70,10 @@
     url-prefix(chrome://) {
 
 [dactyl|highlight~=Bell] {
-    -moz-appearance: none;
+    -moz-appearance: none !important;
+}
+window[dactyl|highlight~=Bell] > * {
+    visibility: hidden !important;
 }
 
 [dactyl|highlight~=CmdLine] {