Open your Task-Manager and tell us the CPU-stress in percent you get there on the process of Firefox.

Ctwx said

When accessing a Twitch stream Firefox needs much time to finish loading and sometimes even freezes for some seconds. It lags a lot. I am not sure why this happens. This is on my Notebook (Acer Aspire ES1-311-P1D5) which is not the fastest one, but those sites work fine in Edge. I am running Windows 10 x64 using Firefox 51.0.1 x64. What can I do?

Same problem here. I don't use flash and don't want to use it. BTW Chromium works fine on twitch without any plugin. I lost hope that some day Firefox developers will fix this flaw.