Why doesn't http2 work in Firefox (nginx, NPN)?

Question

Nginx

There is a backend with php, there is a "balancer". Both on CentOS 7, fresh nginx everywhere. The balancer terminates SSL, and works with the backend through proxy_pass over http1. HTTP2 is enabled in the balancer settings.
Never attached importance to http2, but accidentally decided to check and was surprised when I saw HTTP / 1.1 in the headers. The server doesn't support ALPN, so it shouldn't work in Chrome, but why are there problems with Firefox?
SSL settings taken from here (Intermediate): https://mozilla.github.io/server-side-tls/ssl-conf...
SSL Labs gives an "A" rating.
Test https://tools.keycdn.com/http2-test gives: Yeah! Server supports HTTP/2.0. ALPN is not supported.
What can be wrong? Or in Firefox http2 now also does not work without ALPN?