Your website doesn't have to have "shitty poor design" for it to lag FF. Compare scrubbing in this React player in Chrome and Firefox: https://hsreplay.net/replay/WpWzqzkCHTGdEmbguHUoiC
It comes down to issues in the Firefox JS/DOM engines.
In my experience, Twitter and ESPN can bring any browser to its knees
Your website doesn't have to have "shitty poor design" for it to lag FF. Compare scrubbing in this React player in Chrome and Firefox: https://hsreplay.net/replay/WpWzqzkCHTGdEmbguHUoiC
It comes down to issues in the Firefox JS/DOM engines.