On Firefox 20 beta (till 7) I often notice that my small Asus EEE netbook becomes loud and hot. When you verify in Task manager 50% CPU time for Firefox. It means that one of 2 CPUs is running at 100%. Probably no one would notice on faster many core machine :) I think the CPU usage is not related to content of pages as all of them are static pages without Flash or similar suspects. Just closing the tabs and leaving only one does not decrease the usage. After browser restart it returns to normal state, even if it restores all same pages from last session I've noticed that that problem happens when there is new update ready and in About dialog the "Restart to Update" button is available. Next time it will happen, I can look at thread stacks to identify what code is taking all CPU.
I have also noticed this constantly with Nightly, starting all the way back from Nightly 20.0a1 all the way up to now with Nightly 24.0a1 (2013-06-24). The CPU usage increase isn't very noticeable until a update has been ready for a few hours. Applying the updating and restarting immediately fixes the high CPU usage. At the moment I'm not setup to give any useful program feedback, but if I can find the time I'll try to update with such info. (I didn't want to be that 'me too!' post, but I felt that at least another voice was needed to chime in so that it is known that this wasn't just an isolated case for one person)
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