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I have Firefox and Activity Monitor open and it says Firefox is using 65% of CPU. There are around 20 tabs open. Is there a way to tell which tab is causing the CPU to spike besides manually closing them one by one (which I'm working in and don't want to do).

Is it possible for me to tell Firefox to notify me when a tab is taking more CPU cycles than normal? I'm aware of the script execution dialog and I appreciate it, but this is more about finding the page using heavy resources. Maybe like a flag in the tab like the music/video is playing icon that shows in tabs.

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I know of no automated way to get what you want, but this will work as a manual method.

Use Activity Monitor, in Applications/Utilities.

Any web browser will show as just a shell app, with each open page as a child process, so if you go to View menu & select 'All processes, Hierarchically' then click the CPU tab & sort the Process Name column alphabetically, then when you scroll down to Firefox you will be able to see all its child processes indented & quickly analyse which is using most CPU.
You ought to be able to tie this to a specific tab by its name.

Note that the CPU usage in that section of Activity Monitor is "per core" so 100% is "all of one core". You need to divide by the number of cores [real & virtual] that you have to work out the overall CPU% for any individual process.

If you are uncertain how many cores you have, the simplest way to find out is Window menu > CPU History [or Cmd ⌘ 3 ] then simply count the stripes.

Example showing Safari...
[I don't have Firefox, but it should be very similar].

Note that the last 2 visible processes, SafariCloud... & SafariNotification... are not child processes; as they are back at the non-indented level with Safari itself [ignore the little 'lego' icons, use the left-justification of the process name itself to determine indentation.
Also note you can check which view type you are seeing by the Title bar of the window.

Example Core count...
[Don't be surprised if you only see 4 or 8, this is a 12-core + Hyperthreading machine, so it shows 24]

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  • Thanks. I closed numerous tabs because all web pages were running slow. The next time it happens I'll walk through this process. Oct 24, 2016 at 22:22

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