I quickly glanced the thread and don't see whether the results change when running in safe mode: [[Use Troubleshoot Mode in Firefox]] or in a separate, new, clean profile. People forget the least optimized parts of the whole experience are a) the actual web content shown, b) any add-ons installed and touching that content. So make sure the resource consumption is not actually caused by things like Tampermonkey scripts coming from random sources, or a YouTube tab that Google runs some crazy A/B test in sending the render down a loop of untested code on their side… Anything interesting in [[Task Manager - see whether tabs or extensions are slowing down Firefox]]? Being an open source, you can always pitch in, join the perf channels, and capture a profile on your side — and perhaps even trace the issue to some event, paint, timer etc. if you see anything out of the ordinary. See: https://profiler.firefox.com/docs/