The manifest.json here contains an example for an add-on with a popup, you can use for testing.
To access it as a normal browser site, click the "Manifest address" link and change the URL to look like this one:
moz-extension://<ADDON-ID>/examples/popup/popup.html
You can adjust the popup.css file to change the size of the popup for testing.
const sizeCollection = getDebugData(); // function in EnvironmentDetector
console.log(sizeCollection);