Simply:
The API Gateway responds with HTTP/2
but Firefox doesn’t support it by default (UPDATE: for HTTP sites, no for HTTPS).
I would prefer to change the Service response than try to convince all the my Firefox user (imaginary for now 
) to enable that.
And, in any case, the user agent is requesting POST / HTTP/1.1 and, not sure about this, but I suppose the server should make it’s best effort to answer in that version.
So, Does anybody know how to do that? or, what are your toughs about this issue?.
UPDATE:
Of course, another way to handle this is to make the app run from an https site. I was facing this while developing, and I was using http during that, so the only “user” for now is me, or any developer who prefers to use only HTTP.
So, I think it is no big deal. It is more a problem of forcing HTTPS everywhere.
Thank you.