I dont think there is an easy solution for the lacking performance, but to force yt to use a set qulity you can use the FF addon Enhancer for YouTube (https://addons.mozilla.org/en-US/firefox/addon/enhancer-for-youtube/)
I dont think there is an easy solution for the lacking performance, but to force yt to use a set qulity you can use the FF addon Enhancer for YouTube (https://addons.mozilla.org/en-US/firefox/addon/enhancer-for-youtube/)