I am running Firefox 52.0.1 on Windows 7. I installed the Dom Inspector add-on by Sea Monkey Council and restarted Firefox per instructions. When I attempted to launch Dom Inspector, nothing happened. It would not open a Dom Inspector window. The built-in developer tools window open without a problem. I rebooted but that did not solve the problem. I notice that the Dom Inspector and the Toggle Windows command share a shortcut: Ctrl + Shift + I.