I have working hardware acceleration with the ESR version and media.ffmpeg.vaapi.enabled in about:config, works out of the box.
Did you verify that you get hardware acceleration with a tool like MVP outside of the browser? You can use vainfo and intel-gpu-tools to verify supported codecs and hardware utilization. The intel-media-va-driver-non-free drivers might also work.
Finally, youtube is a good site to test hardware acceleration, Netflix doesn't support hardware acceleration in Linux.