Firefox Is High All The Time
If Firefox is using enormous amounts of CPU resources all the time, not just when visiting specific sites or loading specific types of content, then something else might be wrong with the web browser. One area to consider is extensions. Extensions are not all the same and Firefox may have trouble incorporating extensions developed by different developers, due to different standards and lack of support for Firefox updates.
Some features of your computer's video card or drivers may also contribute to high CPU usage. Specifically, aspects of hardware acceleration, including cursor and bitmap, may cause problems when Firefox displays images. Disabling hardware acceleration may reduce CPU usage.
Check to see if Firefox is running in Windows Compatibility Mode. This is known to cause Firefox to use a lot of CPU. Find the firefox.exe file in C:\Program Files\Mozilla Firefox and right-click it. Select Properties. In the properties window, click the Compatibility tab. The box under Compatibility Mode should not be checked. If it is, uncheck it.
One more area to check is Windows Prefetch. This is used in the boot up process and it may cause Firefox to use more CPU than necessary. To remove prefetch elements, go to C:\Windows\Prefetch\ and delete filenames that begin with FIREFOX.