All of a sudden this morning, Sunday, 1 July, Firefox is driving both cores to 100%, top shows
where Firefox is the top application (and, yeah, right now I'm running logged in as root because there's no browser history, cookies or whatever).

The only add-ons or extensions I have are Adblock Plus and NoScript.

Seamonkey does not display this behavior (it has the same add-ons installed).

Is it just me?

Can you create a new profile and run FF as non root?

Do you use ntp? There was a leap second last night and there is a bug in the Linux kernel which can show like that. You can fix it by setting time once as root:

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Well, well, well, whodathunkit. Thanks for the information.

I knew about the leap second, didn't know there's a kernel bug, had no clue. It was looking like, maybe, a solibs problem ('cause Firefox and, also all of a sudden, Seamonkey -- which did not exhibit the problem but started after roughly a half-hour delay!?). No problem with a couple of other browsers. Humph.

Plus, silly me, I figured that NTP would take care of syncing the leap second (hopefully rolling the clock a little at a time); well, guess not, eh?

And, nothing but the browsers ramped up activity.

All is well that ends.

Here is the original report http://www.sheeri.com/content/mysql-...gh-cpu-and-fix and here is an analysis of what went wrong in the kernel https://lkml.org/lkml/2012/7/1/27.