This seems to be a build-problem (nginx or openssl or both), and sorry i have no solution. In the handshake-phase, firefox can not take the 1.3-Path so it falls back to 1.2. If you tell nginx only to allow 1.3 (not 1.2 fallback), the debug-log of nginx says
2018/10/05 20:19:05 [info] 3021#3021...