Firefox recently starting supporting HTTP/2 and it has stricter requirements than HTTP/1.1. You can set Firefox NOT to use HTTP/2 as a temporary workaround if necessary, but it would be a huge pain to have to have all your users do this.
(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button promising to be careful.
(2) In the search box above the list, type or paste http2 and pause while the list is filtered
(3) Double-click the network.http.spdy.enabled.http2 preference to switch it from true to falsehttps://support.mozilla.org/en-US/questions/1143320