Sometimes, the first time I show the Hook window, it says ‘No valid resource detected’. If I press Escape to hide the Hook window, then try again, it usually works. There seems to be a delay in Hook’s ability to pull usable information from the foreground application. It seems to happen more often when the foreground application has just been started.
MacBook Pro (15-inch, Mid 2010) 2.8 GHz Intel Core i7 OS X 10.11.13 4GB RAM – Doesn’t work for me with Firefox i.e. URLS. Works for files.
Sorry for the delay Jeff. Getting the hang of DIscourse and had not seen that topic. That’s the expected behavior. No external application can detect the URL and name of the resource until the web browser supplies it. And that happens when the page is sufficiently loaded in the browser.
The hardware won’t matter to Hook. It’s the version of macOS that matters. Hook requires macOS 10.12. The latest Download page did not include this information, and for this we apologize. It’s there now. That should also figure in DMG installer.
I’ve prefixed the title with “[invalid]”.
We crafted the wording to deal with various circumstances and what Hook (or any similar app) can know about the foreground resources situation. There is also a status label with a help link that leads to this 
No Resource Detected – Hook