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I am running Firefox 3.6.6 on Windows XP SP2 on a Dual Core. Why does my CPU usage increase up to 100% for one core when I watch movies on

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We'll need more information to answer this question. What addons do you have installed? What OS are you on? Have you tried using a different browser? Have you tried clearing your cache and cookies? – Hello71 Jul 16 '10 at 22:10
Can you post your computer specifications too? For very old computers this tends to be normal behaviour, for almost old to new computers this shouldn't occur. – Tom Wijsman Jul 16 '10 at 22:13
Just guessing, its Flash doing it. – Moab Jul 16 '10 at 22:37
I'd say your core is just a bit underperforming for this purpose. By the way, I've noticed that lateley flash apps and videos on my box are eating more CPU than they used to, say, 6 months ago – U-D13 Jul 16 '10 at 22:43

I have noticed that some versions of flash seen to do this. You might try upgrading to the latest and greatest version to see if it helps. Another thing that might affect this is the quality of the video you are watching. Many older systems just don't have the power to render decent video.

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