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I am running Firefox 12 on a Macbook Pro, and it keeps going up to 99% CPU usage, I can hear my computer revving up and feel it getting hot. The previous version did this as well.

I don't have any add ons and only the basics Flash etc. extensions; I have a Java 2 extension but it is disabled. I don't want to damage my Macbook Pro. I stop and restart Firefox, empty the cache etc but it keeps happening.

Does anyone have an idea why this is happening and how to fix it?

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Check your Activity Monitor located in the Utilities Folder. Many times it will show that a Firefox Plugin Process (shockwave Flash) or other plugin process is pegged at 99%. Depending on the website you are viewing (playing Video usually), this could be the problem. Kill it with the red "quit process" button in the upper left hand corner of the Activity Monitor. You can easily determine which sites are causing the excess usage through this process.

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Try to deactivate Flash and then all other still active plugins one by one. Restart Firefox and see if the CPU is still on 99% after each deactivation.

There's also a page on Mozilla that has some tips: Firefox consumes a lot of CPU resources

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In addition, does it happen on specific websites? If so, these could be rogue scripts that stall the browser. If on youtube, try to use html5 ( assuming that Flash is causing this. – lupincho May 1 '12 at 7:48
I notice it when flash ads come up on some sites. Others can have flash and it doesn't go up. I will test the plugins – pixie289 May 1 '12 at 23:16
ok I checked all my extensions and all my add ons and even with all of them disabled and in safe mode firefox still spikes to 103. It does it a lot when I go to any of the ist. sites like – pixie289 May 3 '12 at 1:08

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