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I am having a Firefox with plenty of add-ons installed. Sometimes Firefox get stuck with high CPU usage on the pages having no Flash. I'd like to solve what's causing this in a more professional manner than randomly disabling extensions, as this method is unreliable, since the high CPU usage cannot be reproduced in a reliable manner.

Is there a way to produce a thread-dump out of Firefox to see what threads there are running and where they spend their time?

Platform: Mac OS X (if Firefox doesn't have a platform neutral tool here...)

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up vote 3 down vote accepted

Debugging instructions are at https://developer.mozilla.org/en/Debugging_on_Mac_OS_X , but before you spend a lot of time on it, review http://support.mozilla.com/en-US/kb/Firefox%20consumes%20a%20lot%20of%20CPU%20resources - debugging is a slow tedious process, and other methods are likely to pay off more quickly:

First off - determine if it's an extension at all by seeing if it happens in safe mode. Second, if it is an extension, because most extensions are interpreted code, it's going to be difficult to distinguish from a debugger or thread-dump which extension is causing it. Process of elimination remains your best bet - not randomly disabling, but systematically working through the possible suspects. Finally, there are a few "usual suspects", documented across various knowledge bases, forums, and mailing lists. http://kb.mozillazine.org/Problematic_extensions is one such list. Mozilla tries to automatically identify ones causing slow startup time at https://addons.mozilla.org/en-US/firefox/performance/ too - while slow startup time doesn't guarantee that it's also a CPU hog while firefox is running, there's enough correlation that it's worth checking the addons listed first.

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