Thank you for the review. That's unfortunately impossible from a technical point of view to save a moz: page. I do not know what moz: page you want to save but if that's the reader view, note that there is a button in the annotation editor to format the page like in the reader view. If you have an account on github, I recommend to open an issue if you want to discuss about this.

Reply #2
THIS page cannot be saved with Firefox because Mozilla blocks extensions on some of their domains. You can use another browser like Chrome to circumvent this limitation though.

Thank you! Fortunately, HTML is already a standard :). MHTML is almost dead, Google will probably withdraw its support on Chrome in favor of... Web Bundles (https://web.dev/web-bundles/). Mozilla said they do not want to implement this new specification in Firefox because of the potential security issues it would introduce, see https://docs.google.com/document/d/1ha00dSGKmjoEh2mRiG8FIA5sJ1KihTuZe-AXX1r8P-8.