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CookieSafe Firefox Add-On and 100% CPU load?
In windows configurations, it seems the actual problem comes from the "Use Strict Rules When Testing Exceptions" setting.
When this option is unchecked, Firefox behaves without noticeable depreciation of speed.
Tools>Add-ons>CookieSafe>Options>Behavior>Exceptions> and now UNCHECK "Use Strict Rules When Testing Exceptions".