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Sometimes there's a nasty, guilty, CPU-hogging tab in Firefox that's maxing one of my CPU cores and making the whole browser sluggish. I'd like to find it and close it.

Sometimes I get lucky, but not often enough. So even the slightest hint of which tabs are most active would be a great help. (That is, even without a Chrome-like process-per-tab model or 'Task Manager', any idea that a particular tab is over-active would be appreciated. Is it using a lot of JS timers? Triggering a lot of events/invalidations? Etc.)

Already have removed Flash. Already run with NoScript (which usually helps but occasionally, its partial blocking of a page seems to trigger the busy-loops). Fingering the top memory-users via the "about:memory" report is slow, awkward, and doesn't quickly find the real culprit.

So: is there any other developer feature that could help? Or an add-in? Or a third-party tool (perhaps based on attaching to FF like a debugger) that can 'blame' a tab for whatever pointless spinning is happening?

(Looking for a way to finger the tab or tabs most likely to be responsible. Not interested in solutions that require a Firefox restart each time the problem recurs: that already works but loses too much loaded data.)

(APPEAL FOR REOPEN: By being more specific about things tried and non-responsive answers, this highly-findable question can draw more directly-responsive answers, like the about:performance answer hidden in @Thomee's comment and also at @fmt's answer on another question. Then as a true answer here, I could then 'accept' it for added findability. As it stands, the duplicate notice just points to yet another locked-as-duplicate question that also can't get the best current answer added/accepted.)

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    Take a look around in about:about, you will find some helpful utilities in there, along with one about memory usage (though i do not remember if it contains information broken down by-tab). – HalosGhost Jun 27 '14 at 4:01
  • this is not possible until electrolysis (separate each tab into a new process) is fully working in Firefox. – magicandre1981 Jun 27 '14 at 4:01
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    Unfortunately none of the answers there (including the accepted answer) offer any help with identifying Firefox tab CPU usage, the unique thrust of this question. (They only address memory.) Other similar questions have the same problem, or include other outdated suggestions for half-fixes. I'm still hoping for a fully-responsive answer, perhaps after Mozilla or some add-on-maker finally offers a reliable window into tab-busyness. – gojomo Feb 24 '15 at 2:13
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    This question is still the top google result for "firefox tab cpu usage", so after digging elsewhere, I'll add this here: Firefox now has an about:performance which gives exactly this information. – Thomee Jun 22 '16 at 16:51
  • Sometimes, if you're lucky, the CPU hogging tab is also hogging memory. Then you can identify it. – Bryce Sep 1 '16 at 17:35
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As you can see here it's on their list for 7 years.

Electrolysis is available in the nightly version roughly since the beginning of 2014, and that seem like the only possibility at the moment.

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Via a suggestion from HN user ~liminal, a short sampling (~10 seconds) using the Developer Tools 'Profiler' might be helpful in such situations.

I've just tried this in Firefox 33.0, and many of the report-rows include a related source domain, which may help identify the tab responsible. (Unfortunately, many of the report-rows are related to domains like apis.google.com, which might be in use in any given tab.) Still, it's a start.

  • "many of the report-rows include a related source domain" - I'm not seeing this at all, FWIW. – tbone Mar 15 '15 at 17:52
  • After toggling the recording on and off, the call-timings info comes up. That takes a while, and even seems to hang if I try more than a few seconds of sampling. But if it does come up, then each of the call-frame lines has a domain-name in light-gray text on the far-right. (If you're not seeing the timing table at all, maybe try a much shorter sampling period? FYI, here's an example of what I see from a very brief profiling run: imgur.com/kZTBCpE ) – gojomo Mar 15 '15 at 19:33
  • How to open the Development Tools for the browser (not for a single page)? – Monsignor Apr 5 '17 at 11:13

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