I've got a Firefox process with nothing in it but the Google home page (the "search" page, not the "homey" home page with news etc.) and this site. I've killed it and restarted it multiple times. It's burning massive amounts of CPU cycles doing ... what?
I notice that Firefox, over time, leaks memory terribly and uses more and more CPU, but not anything close to this much. What is it doing? At present it's the primary process loading down the system, and that's new behavior. Eating memory and getting bloated is usually something that makes it sluggish. Now it's just hogging resources and spinning my fan.
I'm running 12.04 on an HP laptop but that shouldn't matter of course.
Are there some Firefox tools for looking at its internal thread activity or something?
edit newly-created profiles show exactly the same behavior ...
This LKML thread describes a kernel issue that appeared due to the 2012 leap second insertion (1 Jul 2012). I don't know exactly what they're talking about, but it seems that the clock update didn't do some internal bookkeeping, with the result being that some synchronization calls weren't resulting in the ordinary process blocking that they'd cause. Thus, some user-mode processes that rely on the kernel service were just spinning the CPU endlessly.
The fix, however, is extremely simple:
Immediately after doing that — and I did nothing else — the Firefox process calmed down. I could see the sharp "cliff" on my CPU monitor.