Firefox CPU usage
From MozillaZine Knowledge Base
This article describes how to prevent or work around high CPU usage in Firefox.
Only on specific pages
If the problem happens when viewing PDF files in the browser using the Adobe Reader plugin, disable the Adobe Reader plugin (see Issues related to plugins for details) and either use Adobe Reader as an external application, use another PDF viewer or else use the built-in Firefox PDF viewer.
Report the site
Report the site using Help -> Submit Feedback. Reporting the problem will help identify sites with problems so Mozilla evangelists can help fix their problems, and also help Mozilla developers track down any bugs in Firefox that could be causing excessive CPU usage.
All the time
Problematic extensions can cause excessive CPU usage.
Windows Compatibility mode
Firefox can cause excessive CPU usage if it's set to run in compatibility mode for an older version of Windows. To fix the problem, right-click on the Firefox icon you click to start Firefox, click Properties, click the Compatibility tab, make sure the check box under Compatibility mode is unchecked, and restart Firefox.
Disabling cursor and bitmap hardware acceleration on some video cards and drivers can reduce CPU usage when displaying images. To check hardware acceleration on Windows, right click on the desk top to get the display properties menu. Then select Settings > Advanced > Troubleshoot. This article shows how. For more detailed information on troubleshooting display properties, see this article.
In Windows XP and above, deleting the Windows prefetch files for Firefox (C:\Windows\Prefetch\FIREFOX*) can prevent problems with the jpeg_fdct_islow thread consuming lots of CPU.  Note: Automatic deletion of FIREFOX.EXE* prefetch files is planned for Firefox 15 (bug 692255).