The '''moz-extension:''' protocol refers to a file added by one of your extensions that uses the new WebExtensions interface. Unfortunately, this is difficult to investigate because the address does not correspond directly to the the extension's true ID on the troubleshooting information page; it is intentionally different so websites cannot guess it. Therefore, it's almost certainly an "internal" reference triggered by the extension itself, rather than by web page content. Given the name, I suspect it's just a blank placeholder page.