title: Firefox debugger blank without JS sources
body: <p><a href="https://i.stack.imgur.com/wlfOs.png" rel="noreferrer"><img src="https://i.stack.imgur.com/wlfOs.png" alt="Screenshot of Firefox debugger"></a></p>
  
  <p>My version of Firefox Quantum is 63.0.3 (64-bit) on Windows 10. When I open debugger, I can't see any source files. I can confirm that JS is enabled and there are JS files on the site/page. </p>
  
  <p>Perhaps, there's some setting that I need to correct.</p>