@Noah_SUMO: Thank you, but I've already tried disabling all plugins in Fx's safe mode even when booting into system's safe mode. I tried multiple times.

There were more than 100 processes running because I was writing a software document, so I had word, dictionary, multiple im, code editor, download managers and music player running, as well as a cup of coffee. In windows safe mode there are about 40 processes running but I still encountered the very problem.

I'll keep using Fx but just remember not to leave it minimized and everything will be fine.

My last piece of advice would be to check out the Process Monitor tool:
http://technet.microsoft.com/en-us/sysinternals/bb896645.aspx

It really is a amazing tool. It can allow to you see what any program is doing in the background. Such as what files it is accessing or writing to. If you want, you can run this tool as soon as you notice Firefox's CPU increasing. It may take a little work to filter out the noise from all the other programs b/c you will see ALOT of information flying at you. But if you can create a filter and customize it to ignore other programs and just focus on firefox.exe, you should be able to see what files it is accessing. And hopefully that will pinpoint whatever the problem is.

If it helps you track it down, please let us know what it was. Thanks!

Good idea, I forget about super process monitor lying in hdd for years...will try it asap.

@Noah_SUMO: unfortunately, nothing abnormal happens to firefox.exe when it's minimized except cpu usage boost. TCP sending and receiving frequency keeps steady, with occasionally reading files in profile folder and reg query for net interface.

Firefox 21 still suffer from same problem...

This happens on my Mac too, starting with the last update or two (FF 20-21, somewhere in there). FF is constantly using 15-20% cpu with about 40 threads for only a single page loaded in the background (when I'm not actively using FF). If I sit and watch the Activity Monitor (like the Task Manager in windows) without touching my computer, FF will sometimes jump up to 40%.

So, I doubt the issue here is malware.

Maybe try to change this perf http://lifehacker.com/5161563/prevent-firefox-from-hogging-memory-when-minimized

@colbabomb: there is no config.trim_on_minimize in recent fx builds and it doesn't seem to work.

you have to create it by right clicking blank space in about:config then select new boolean

I am not sure the pref

• config.trim_on_minimize

is going to be of much help with this problem, because it

1. relates to memory usage not CPU usage
2. it is going to have an effect on applications or add-ons that Firefox is using, but I understand the OP sees the problem in safe mode and with addons disabled.

• for an explanation of this pref see
  http://kb.mozillazine.org/Config.trim_on_minimize

Note if you do have memory problems relating to Firefox a mass of dynamic data may be obtained from about:memory

Profiler

It may also be worth trying to use the Firefox profiler to see if that generates any useful information. The profiler is a new feature, and I have not really tried it out myself, but it who knows it may throw some light on the situation.
Not sure what if any documentation there may be but

• see https://developer.mozilla.org/en-US/docs/Performance/Profiling_with_the_Built-in_Profiler
  The article is old on my Firefox I may open the profiler with Tools -> Web developer -> Profiler or shortcut Shift + F5
• see also http://benoitgirard.wordpress.com/2012/07/27/javascript-profiling-with-the-gecko-profiler-and-js-anti-pattern/

Not sure what the OP's problem is, and I do not think anyone has been able to recreate / reproduce this problem.

Firefox Portable

It may be interesting to see what happens if Firefox portable is installed. That may be easily installed as an additional Firefox browser. So it is one way of getting a fairly clean install of Firefox that runs in the same environment, but independently of the main Firefox browser.

Yes I understand it is for memory (RAM) usage but could possibly help, anything is possible.

@colbabomb: yes I created the key, but neither RAM usage nor cpu hugging when minimized changed.

@John99: OK I'll try that, thanks.