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

lib/inspectorObserver.js

Index: lib/inspectorObserver.js
===================================================================
--- a/lib/inspectorObserver.js
+++ b/lib/inspectorObserver.js
@@ -43,20 +43,28 @@ let InspectorObserver =
 
     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", () =>
     {
@@ -78,13 +86,13 @@ let InspectorObserver =
         });
       }
     }, 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();