Not OP, but what I think is that FF renders the twitch.tv page very slow. On macOS, not just the tab but the whole FF app locks up, showing a spinning beach ball. It's the only site that I found that does that.
I don't personally visit twitch.tv but I just did and it loaded normally and streaming auto-played without enabling Flash (although the site requested it for some reason).
Speaking of Flash, some sites request Flash on Firefox but not on Chrome for example (although it's totally unnecessary). This is becoming less frequent recently but it shows that a lot of websites are not taking advantage of Firefox's full features as they do with Chrome (something to consider as well).
