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, it's likely a problem with the Adobe Reader plugin. One possible fix is to create a "WWW_OpenURL" directory in the "AdobeUM" directory.   Another workaround is to disable the browser plugin so that PDF files open in Adobe Reader as an external application, as explained here.
Report the site
Report the site using Help -> Report Broken Web Site. 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
After Standby or Hibernate
There are some bugs in older versions of Firefox and Flash that can cause Firefox to consume lots of CPU after the computer has been in standby or hibernate mode. These can be fixed by updating to the latest Flash player and either Firefox 126.96.36.199 or Firefox 3.
- Bug 213637 – Mozilla runs at ~100% cpu usage after connection is interrupted or wakeup from hibernate or standby
- Bug 376643 – setInterval fires repeatedly with incorrectly small delays if machine is suspend/resumed, hibernate/resumed or process is SIGSTOP/SIGCONT
- Bug 265172 – Hang and 100% CPU usage sometimes during restore/resume from sleep/standby/hibernate - update your flash player
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.
Deleting the Windows XP or Vista prefetch files for Firefox (C:\Windows\Prefetch\FIREFOX*) can prevent problems with the jpeg_fdct_islow thread consuming lots of CPU.
- Firefox consumes a lot of CPU resources at support.mozilla.com