Maybe because Firefox is stricter? Netlify (incorrectly) sends a 4,583 byte chunk, while GitHub (correctly) sent precisely 1024 bytes. Chrome might just trim it to 1024 bytes, but Firefox just fail-safe at the difference.

Edit: Netlify is indeed wrong: asserts in the headers that the content length is 1024 bytes but sends up 4,583 bytes of content. That will definitely fail in Firefox.