What inside Firefox is using my CPU ?

Submitted by dag on Thu, 2009/06/18 - 00:54

I love Firefox a lot, more than any other browser actually. With my small set of addons, me and Firefox, we conquer the world every day !

That's why I hate it when my buddy Firefox is chewing away too many CPU cycles while not doing anything. It is really frustrating because it heatens my lap(top), flattens my battery and slowens my work. (yeah yeah, I know that is not proper English)

I tried all the usual suspects, Google mail and calendar, the obvious heavy website. I closed many tabs, but I never seem to find the culprit. Whatever I end up closing, 100% of one CPU remains going down the drain. Restarting Firefox is no solution either.

Anyhow, I discovered that when this happens, it always is Javascript related. Because the trick is to disable Javascript. (And then simply enable it again because I am an addict !)

So why is there no simple addon that can tell me which of my 100+ tabs is only taking and never giving ? And why, if it does exist, did nobody notify me of this addon ? :-)

Let's hope Firefox 3.5 fixes the problem and not simply uses 100% CPU more efficiently ;-)

Hint: My favorite addons are: Flashblock, Tab Mix Plus and Uppity. I don't need much more...


have you tried disabling those plugins? or starting firefox with a squeaky clean profile?

as far as javascript is concerned; i use the great noscript plugin to block javascript expect for the sites i put on a whitelist. it's a drag sometimes, but from a security (and indeed maybe performance) point of view a real plus.

I guess it's impossible

Unless you're using FF 3.5, I don't see by what means could an addon tell which tab is using how much of the CPU. They're all in the same process.

Otherwise, Javascript is the #1 idiocy of the IT. (#2 being Java, #3 being .NET and Mono, #4 being Macromedia/Adobe Flash, #5 being Silverlight and Moonlight...)

welcome to the 21st century.

welcome to the 21st century. IT is no longer about fortran et al.

you obviously must find object orientedness suspicious then...

Try the NoScript plugin

You might want to try the NoScript plugin.

It's not exactly what you are after, but by enabling scripts individually you might identify the offender.

Clean your profile

Sometimes it helps to delete the .mozilla folder, and start anew with a clean profile.

Just re-install the addons and Firefox is performing well again.

Remember to backup your bookmarks, if any.


Dag, maybe it's the peril RMS warns us about:


The good news is that the soon-to-come Firefox 3.5 has an entirely new Javascript engine; actually, it's its main new feature...

Best regards,

forum sites

I've noticed the same thing on a few web pages. Many of them tend to be forums (of the vBulletin variety) that seems to have some javascript that runs away. I'm not sure if it's from some bad advertising, or just something that runs in the forums themselves. I suspect it is the forums themselves as I do use noscript and only allow the forum site itself. It's some kind of AJAX problem.

I keep several browser

I keep several browser profiles for this sort of thing and start Firefox with "firefox -ProfileManager" or "firefox -profile myhtmlonly" when I need a specific one. It's pretty handy for all sorts of things. Another similar approach is to run Epiphany or Konqueror at the same time as Firefox but with different settings.

When I use Firebug it happens

When I use Firebug it happens what you refer, but for me is an expected behaviour when debuging the JavaScript.

Check if you have any plugin debugging every JS instruction, usually that is the problem.


Usually I can spot a site by going through the tabs and looking at the status message bar. Although there is no change in the appearance of the tab, the status message is changing every couple of seconds. I think it's just poorly written or overly aggressive scripts, usually to an ad site, or google. The addon, No Script, or Web Developer to stop javascript completely, might be the only solution. I'm thinking of having two profiles for Firefox. One, with all the fun stuff, and one for work. The one for work, won't enable javascript.

It's facebook.

It's facebook. Seriously.

Close all the facebook tabs; you'll see.

Microsoft Plugin

If you look at your Plugins..and you see a Microsoft type plugin..DISABLE IT!. My CPU percentage dropped from 80% usage to 6% - 15%. on the average. I've also known Microsoft fonts be a disaster and other little things they install onto your system (I'm on Mac)

I think options for diabling

I think options for diabling JS should be introduced, by default for background tabs user idle for more than 5 minutes. Pinned apps or an exception list should be allowed as exceptions. A right click option for each tab as an override. But if I havnt used a tab in 5 minues, more than likly (excpet rarely) I dont want it doing anything at all thank you! Exceptionsa re gmail (and other pinned apps)

A monitor on tab cpu usage would be nice.

Microsoft Plugin

If you look at your Plugins..and you see a Microsoft type plugin..DISABLE IT!. My CPU percentage dropped from 80% usage to 6% - 15%. on the average. I've also known Microsoft fonts be a disaster and other little things they install onto your system (I'm on Mac)

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