User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:188.8.131.52) Gecko/20070502 Firefox/184.108.40.206 Flock/0.8.0.99
I noticed this on http://abcnews.go.com/, they have a news ticker that is driven by animation code (http://a.abcnews.com/assets/js/animation.js), where they use a setTimeout of 10 ms to update the animation. Even while sitting in a background tab, the browser consumed 50% of CPU while idle.
To mitigate this, the browser could keep track of where and how often timeouts are scheduled, amd throttle down content that tries to schedule things too often.
Steps to Reproduce:
Bug 633421 fixed this.