Can I DevTools?
  1. Open Devtools Settings (F1) > Under Advanced Settings, enable "Enable browser ... toolboxes" and "Enable remote debugging".
  2. Open Firefox Menu (via hamburger icon) > More tools > Browser Toolbox. You’ll be able to inspect the browser’s chrome windows and see, and be able to debug, all the JavaScript files loaded by the browser itself and by any add-ons that are running.