Firefox CPU usage
This article describes how to prevent or work around high CPU usage in Firefox.
Only on specific pages
Plugins
If you're only having problems with sites that use Flash and updating to the latest Flash version doesn't help, you can use the Flashblock extension to selectively enable or disable it [1][2]
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 [3]. Another workaround is to disable the browser plugin and so that PDF files open in Adobe Reader as an external application, explained here.
JavaScript
Some sites can cause high CPU usage through extensive use of JavaScript. You can disable JavaScript selectively using the NoScript extension.
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
Extensions
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.[4] 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.
Hardware acceleration
Disabling cursor and bitmap hardware acceleration on some video cards and drivers can reduce CPU usage when displaying images.[5]
Windows prefetch
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.[6]
Related bug reports
- Bug 213637 – Mozilla runs at ~100% cpu usage after connection is interrupted or wakeup from hibernate or standby
- Bug 265172 – Hang and 100% CPU usage sometimes during restore/resume from sleep/standby/hibernate - update your flash player