Firefox browser validation reports amp errors vs chrome validation is successful


## Difference of validation Success vs Fail
Following the News Article tutorial, I used VS Code and the live server extension opening firefox as well as the Chrome Server extension on macOS resulting in a validation success on Chrome but failure on Firefox

## How do we reproduce the issue?
[News Article convert HTML to Amp tutorial](https://amp.dev/documentation/guides-and-tutorials/start/converting/?format=websites)
1. Follow the tutorial as laid out step by step
2. Run the site on both chrome and firefox side by side
3. Try using The AMP Validator from the link within the errors on the firefox console.

## What browsers are affected?

*Firefox: Powered by AMP ⚡ HTML – Version 2104170104001 http://127.0.0.1:53452/article.amp.html#development=1
The AMP Validator reports that it "could not fetch the given URL"
Validation Status: FAIL

Chrome: Powered by AMP ⚡ HTML – Version 2104170104001 http://127.0.0.1:8887/article.amp.html#development=1

Reproduce the News Article convert HTML to AMP tutorial in Firefox and Chrome, using the AMP Validator link from the Firefox console. Compare the local URLs, server setup, and validator output; done means identifying why Firefox reports “could not fetch the given URL” or documenting the browser-specific limitation.

Written by the indexing model from the issue text.