browser using CPU 100%
 

I have a quad-core Intel chip and I find that either Opera or Fire Fox browser uses one of the cores 100% of the time, even when it's idle and minimized. I slows down the whole system. I'm not sure but I don't think it always worked this way. This is slackware64 14.1

When I run top I see the the process running full time is 'X'. If I kill X, I will drop to console mode. Restarting X resumes the problem. If I quit the browser the processor usage drops down to 10-20%.

First of all, do you have any extensions or apps that could be doing something nasty (like decrypting some files in your home folder)?

If not, I would report it to the Mozilla Bugzilla.

Got a quad Xeon myself, notice nothing of the sort, so maybe it's specific to your configuration.

No, I'm running a standard system as far as I know. I have been running dolphin, plasma-desktop, opera, fire fox, gimp, gwenview and other apps included with the distro. The problem doesn't seem to be specific to the browser but does occur when one or both are running. Currently (after killing the plasma process and restarting X) I am not seeing this problem, even as I am using opera to post this message.

Could have been Java in the background loading ads. Try installing an ad blocker and see if you ever get it again.

I may have stumbled on the solution. I killed the plasma-desktop process then quit 'X' and since I restarted X it has been working normally. My method is awkward and there must be a better way to achieve the result but killing plasma-desktop seems to have been the magic.

You could try using a different desktop such as GNOME, MATE, Trinity, etc. and see if you have the same issue. It could also be in the way that X is started, may be worth a look to check your init scripts.