Several times, I've noticed when I look at my computer in the morning, when it's been left on overnight, Firefox has high CPU usage (25% on this 4-core machine).
The first few times, I just restarted Firefox, and then it went back to low CPU usage (even though I've got it set to re-open all open tabs from before). After that happening several times, I tried closing tabs one-by-one, and found that it's the StackOverflow tab that was causing high CPU usage. After I've closed the tab, if I re-open it again then it is back to normal CPU usage. So it looks as though there's something about the SO page that causes high CPU usage after it's been open for a long time.
This is on Firefox 13 and 14, on Windows XP SP3, 32-bit. I haven't tried it with other browsers.
Update: I left both Firefox and Chrome on overnight with StackOverflow open. Chrome was fine. Firefox was again on high CPU usage. I found that just clicking reload for the SO page made the CPU usage go back down.