demo.html is a page with embedded iframe loaded via protocol handler defined in this extension's manifest:

iframe src="web+iframeb://demo"

Iframe content should get redirected and loaded from

iframe src="https://bafkreicysg23kiwv34eg2d7qweipxwosdo2py4ldv42nbauguluen5v6am.ipfs.dweb.link/#web%2Biframeb%3A%2F%2Fdemo"

If you click on the above link in Firefox 71, you will see it loads, but there in parent frame (unintended iframe escape).