Is there anything in the current Firefox similar to the Task Manager in Google Chrome? (Shift + Esc)
There is something that occasionally takes up enough CPU to make Firefox unresponsive - but with multiple windows, each with a dozen or so tabs open, trial & error is going to take a while.
about:memory shows Firefox's memory usage details. There's also a button on that page that allows you to minimize memory usage.
The first likely culprit is Flash. Kill Flash - now.
Then it may be rogue Javascript code. Firefox's Javascript Deobfuscator extention lets you watch the count of Javascript function calls :
It is not a measure of CPU usage, but a close enough proxy : find the function with a runwaway number of calls and you will likely have caught the culprit.
Except for already mentioned about:performance, there is a Task Manager addon that meant to be a clone of Chrome's Task manger.
By using Flashblock and Nevercrash, now replaced with FlashStopper and UnloadTab you go a long way into blocking Flash or tabs, preventing CPU waste instead of tracking it. This holds, but less, for memory too. Flash, e.g. Youtube is prevented to start until you click on them, so you may launch them in several tabs and they wait for you to open the tabs. Tabs are uloaded from memory and idle until you reopen them and they are refreshed from the cache (if still available).
Shutting down the Internet for a while also works during the time you don't need it ;-)
Firefox fragments virtual memory so much that it's using an excessive amount of real memory (there is too much unused virtual memory in real memory). One needs to periodically stop and restart Firefox with the same pages. That will defragment its memory and the whole system will run faster by reducing the swapping.
On my Ubuntu system, real memory usage slowly climbs up to 98%. Then it's time to stop Firefox and restart it: it will then use less than 1MB of real memory instead of 3MB.
The same holds for Thunderbird..
The following answer to another question may help you. The answer is written by the user "accolade".
XUL Profiler is an awesome extension that can point out extensions and client side JS gone bananas CPU-wise. It does not work on a per-tab basis, but per-script (or so). You can normally relate those .js scripts to your tabs or extensions by hand.
It is also worth mentioning that Google Chrome has built-in a really good task manager that gives memory and CPU usage per tab, extension and plugin.
Let me add some more info to accolade's answer. As of January 2012, the latest version of XUL Profiler is 1.0.4, released December 2008. It's only certified compatible with Firefox 2.0 - 3.6.*. So it clearly needs a new maintainer.
I wonder if there's any way to force the extension to work on newer Firefox versions.
Also, I wonder if it works well to downgrade Firefox temporarily in order to use XUL Profiler with your existing tabs on a certified-compatible Firefox version.
about:about will list the about pages which may help you.
Relevant to the specific question, as some mentioned already:
about:processes : Memory and CPU usageabout:performance : Energy Impact and Memoryabout:memory --> Measure : In depth Memory analisis
Here around 20% cpu was not accounted for by about:performance .
Turning off "Enable add-on debugging" checkbox in about:debugging got rid of this extra cpu usage.
You could also turn off this add-on debugging in about:config. Set either devtools.chrome.enabled or devtools.debugger.remote-enabled - or both - to off.
For more see about:debugging - Firefox Developer Tools | MDN
Do you have Flashblock or NoScript installed? Especially if not, I'd try to look into the flash-heavy tabs first. I've also seen lots of eBay tabs grind Firefox to a halt, albeit that was a while back (when I was actually using eBay).
