1) Pick a rMBP. Any one.
2) Set display options - "more space" under System Preferences.
3) Run Firefox. Load any website. No extensions; safe mode doesn't matter.
4) 100%+ CPU, fans kick up and run high, system gets hot and slows to a crawl.
Chrome and Safari run totally fine under the same conditions.
It's documented pretty thoroughly in the first Bugzilla link I posted; the second one is an offshoot where they're trying to solve this specific issue. If you do happen to work at Mozilla, would really appreciate your help escalating.
EDIT: I now have a perf file showing significant issues when clicking on a YouTube video. It gives an error when trying to upload it to the site, but if you contact me through email (in my HN profile), I can send it over.