diff --git a/platform/firefox/vapi-background.js b/platform/firefox/vapi-background.js
index 30c31f4ad..62cdf4569 100644
--- a/platform/firefox/vapi-background.js
+++ b/platform/firefox/vapi-background.js
@@ -2623,7 +2623,6 @@ vAPI.toolbarButton = {
     tbb.id = 'uBlock0-legacy-button';   // NOTE: must match legacy-toolbar-button.css
     tbb.viewId = tbb.id + '-panel';
 
-    var sss = null;
     var styleSheetUri = null;
 
     var createToolbarButton = function(window) {
@@ -2651,6 +2650,18 @@ vAPI.toolbarButton = {
     };
 
     var addLegacyToolbarButton = function(window) {
+        // uBO's stylesheet lazily added.
+        if ( styleSheetUri === null ) {
+            var sss = Cc['@mozilla.org/content/style-sheet-service;1']
+                        .getService(Ci.nsIStyleSheetService);
+            styleSheetUri = Services.io.newURI(vAPI.getURL('css/legacy-toolbar-button.css'), null, null);
+
+            // Register global so it works in all windows, including palette
+            if ( !sss.sheetRegistered(styleSheetUri, sss.AUTHOR_SHEET) ) {
+                sss.loadAndRegisterSheet(styleSheetUri, sss.AUTHOR_SHEET);
+            }
+        }
+
         var document = window.document;
 
         // https://github.com/gorhill/uMatrix/issues/357
@@ -2723,9 +2734,16 @@ vAPI.toolbarButton = {
 
     var canAddLegacyToolbarButton = function(window) {
         var document = window.document;
+        if (
+            !document ||
+            document.readyState !== 'complete' ||
+            document.getElementById('nav-bar') === null
+        ) {
+            return false;
+        }
         var toolbox = document.getElementById('navigator-toolbox') ||
                       document.getElementById('mail-toolbox');
-        return toolbox !== null && !!toolbox.palette && document.getElementById('nav-bar') !== null;
+        return toolbox !== null && !!toolbox.palette;
     };
 
     var onPopupCloseRequested = function({target}) {
@@ -2752,19 +2770,20 @@ vAPI.toolbarButton = {
                 toolbarButton.parentNode.removeChild(toolbarButton);
             }
         }
-        if ( sss === null ) {
-            return;
-        }
-        if ( sss.sheetRegistered(styleSheetUri, sss.AUTHOR_SHEET) ) {
-            sss.unregisterSheet(styleSheetUri, sss.AUTHOR_SHEET);
-        }
-        sss = null;
-        styleSheetUri = null;
 
         vAPI.messaging.globalMessageManager.removeMessageListener(
             location.host + ':closePopup',
             onPopupCloseRequested
         );
+
+        if ( styleSheetUri !== null ) {
+            var sss = Cc['@mozilla.org/content/style-sheet-service;1']
+                        .getService(Ci.nsIStyleSheetService);
+            if ( sss.sheetRegistered(styleSheetUri, sss.AUTHOR_SHEET) ) {
+                sss.unregisterSheet(styleSheetUri, sss.AUTHOR_SHEET);
+            }
+            styleSheetUri = null;
+        }
     };
 
     tbb.attachToNewWindow = function(win) {
@@ -2780,14 +2799,6 @@ vAPI.toolbarButton = {
             onPopupCloseRequested
         );
 
-        sss = Cc["@mozilla.org/content/style-sheet-service;1"].getService(Ci.nsIStyleSheetService);
-        styleSheetUri = Services.io.newURI(vAPI.getURL("css/legacy-toolbar-button.css"), null, null);
-
-        // Register global so it works in all windows, including palette
-        if ( !sss.sheetRegistered(styleSheetUri, sss.AUTHOR_SHEET) ) {
-            sss.loadAndRegisterSheet(styleSheetUri, sss.AUTHOR_SHEET);
-        }
-
         cleanupTasks.push(shutdown);
     };
 })();