Chrome browser offers task Manager, when you use Shift+Esc on Windows you can open it and see which process is consuming a lot of resources (‘stats for nerds’ reveals lots of information). Firefox lacks this feature, with e10s under testing, Mozilla has added Task Manager feature to Firefox 40 Nightly, which shows which tabs and add-ons are slowing down the Firefox browser.
To see it in action, visit about:performance from the address bar.
UPDATE November 26, 2011: Mozilla has updated about:performance page in upcoming Firefox 65 to report memory and CPU consumption per tab, read Mozilla makes about:performance as complete Task Manager in Firefox 65
Performance Monitor displays the following information (check the below screenshot)
Jank level User (%) System (%) Cross-Process (%) Activations
On this, Mozilla’s Nicholas Nethercote says” We have some memory measurements reported via telemetry, but they are of limited use and I only look at them occasionally. In comparison, about:memory is extremely useful for investigating individual sessions. I suspect per-compartment CPU accounting would be similar — great when a single user has bad performance, but of little use in aggregate. Plus you can’t report content URLs without violating user privacy…”
For more information, check the bug 674779– add per-compartment CPU accounting.