--- a/devtools/client/framework/devtools-browser.js
+++ b/devtools/client/framework/devtools-browser.js
@@ -713,16 +713,18 @@ var gDevToolsBrowser = exports.gDevTools
       this._browserStyleSheets.delete(win);
     }
 
     // Destroy the Developer toolbar if it has been accessed
     let desc = Object.getOwnPropertyDescriptor(win, "DeveloperToolbar");
     if (desc && !desc.get) {
       win.DeveloperToolbar.destroy();
     }
+    // And in any case, remove the getter set by _registerBrowserWindow.
+    delete win.DeveloperToolbar;
 
     let tabContainer = win.gBrowser.tabContainer;
     tabContainer.removeEventListener("TabSelect", this);
     tabContainer.removeEventListener("TabOpen", this);
     tabContainer.removeEventListener("TabClose", this);
     tabContainer.removeEventListener("TabPinned", this);
     tabContainer.removeEventListener("TabUnpinned", this);
   },