But if you try to load a local resource as localhost in Firefox…
Image

For the sake of completeness:

Firefox contains a security patch which restricts the kinds of files that pages can load (and methods of loading) when you open them from a file:// URL. This change was made to prevent exfiltration of valuable data within reach of a local page, as demonstrated in an available exploit.

More info: https://developer.mozilla.org/docs/Web/HTTP/CORS/Errors/CORSRequestNotHttp

about:config
security.fileuri.strict_origin_policy
change to false