Issue 4255 - Place Element Hiding Helper button in the DevTools toolbar if one exists

lib/inspectorObserver.js

 /*
  * This Source Code is subject to the terms of the Mozilla Public License
  * version 2.0 (the "License"). You can obtain a copy of the License at
  * http://mozilla.org/MPL/2.0/.
  */
 
 let InspectorObserver =
 {
   init: function()
   {
@@ skipping 30 matching lines @@
     let stringBundle = Services.strings.createBundle("chrome://elemhidehelper/locale/global.properties?" + Math.random());
     let result = stringBundle.GetStringFromName("inspector.button.tooltiptext");
 
     Object.defineProperty(this, "inspectorButtonTooltip", {value: result, enumerable: true});
     return this.inspectorButtonTooltip;
   },
 
   inspectorReady: function(eventName, toolbox, panel)
   {
     let panelWindow = panel.panelWin;
-    let inspectBtn = panelWindow.document.getElementById("inspector-breadcrumbs");
-    if (!inspectBtn)
+    let inspectorToolbar = panelWindow.document.getElementById("inspector-toolbar");
+    if (!inspectorToolbar)
       return;
 
+    let insertBefore = inspectorToolbar.firstChild;
+    let inspectorAddButton = panelWindow.document.getElementById("inspector-element-add-button");
+    if (inspectorAddButton && inspectorAddButton.parentNode == inspectorToolbar)
+    {
+      // Firefox 48+
+      insertBefore = inspectorAddButton.nextSibling;
+    }
+
     let tooltiptext = InspectorObserver.inspectorButtonTooltip;
     let button = panelWindow.document.createElement("toolbarbutton");
     button.setAttribute("id", "ehh-inspector-toolbarbutton");
     button.setAttribute("class", "devtools-toolbarbutton");
     button.setAttribute("tooltiptext", tooltiptext);
     button.setAttribute("tabindex", "0");
     button.addEventListener("command", () =>
     {
       let node = panel.selection.nodeFront;
       let target = panel.target;
@@ skipping 11 matching lines @@
           panelWindow.openDialog("chrome://elemhidehelper/content/composer.xul",
               "_blank", "chrome,centerscreen,resizable,dialog=no", response);
         });
       }
     }, false);
 
     //Override button style for light DevTools theme
     let style = panelWindow.document.createProcessingInstruction("xml-stylesheet", 'href="chrome://elemhidehelper/skin/devToolsOverlay.css" type="text/css"');
     panelWindow.document.insertBefore(style, panelWindow.document.firstChild);
 
-    inspectBtn.parentNode.insertBefore(button, inspectBtn);
+    inspectorToolbar.insertBefore(button, insertBefore);
   }
 };
 
 InspectorObserver.init();