Worth checking the browser console. If you use a content blocker (e.g. uBlock Origin) it may block things the site really, really "wants". It then goes into an extreme busy loop and throws exceptions to the console like a berserker on a nightly rampage.

Once you carefully whitelist the resource it wants the CPU usage goes down again. At least that fixed it for me (I was mistakenly checking various codec related settings in Firefox first too to find a solution.)