#5 hide pinned tabs in Firefox 51+

index.js

@@ -1,17 +1,34 @@
 'use strict';
 
-let { viewFor } = require('sdk/view/core'),
+let {viewFor} = require('sdk/view/core'),
     windows = require('sdk/windows').browserWindows;
 
+const HIDE_PINNED_TABS_STYLE = '.tabbrowser-tabs[positionpinnedtabs] > .tabbrowser-tab[pinned] ' +
+    '{ position: relative !important; }';
+
 
 // listener function
 function addListener(lWindow) {
     let window = viewFor(lWindow);
+    let document = window.document;
+    let toolbox = document.getElementById('navigator-toolbox');
+
+    let styleElem = document.createElementNS('http://www.w3.org/1999/xhtml', 'style');
+    toolbox.insertBefore(styleElem, toolbox.childNodes.item(0));
+    let styleSheet = styleElem.sheet;
+
+    window.addEventListener('fullscreen', () => {
+        let active = window.fullScreen;
+
+        toolbox.style.height = active ? '0px' : 'auto';
+        toolbox.style.overflow = active ? 'hidden' : 'auto';
 
-    window.addEventListener('fullscreen', function () {
-        let toolbox = window.document.getElementById('navigator-toolbox');
-        toolbox.style.height = window.fullScreen ? '0px' : 'auto';
-        toolbox.style.overflow = window.fullScreen ? 'hidden' : 'auto';
+        // hide pinned tabs, required since Firefox 51
+        if (active) {
+            styleSheet.insertRule(HIDE_PINNED_TABS_STYLE, 0);
+        } else {
+            styleSheet.deleteRule(0);
+        }
     }, false);
 }