Firefox uses too many CPU resources - How to fix

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At times, Firefox may require significant CPU resources in order to download, process, and display web content. If you are experiencing periods of sustained high CPU usage while using Firefox, this article presents some options for you to review.

  • The CPU (Central Processing Unit) is the "brain" of the computer. When the CPU is being heavily used, the overall performance of the computer can be impacted.
  • Depending on your operating system, you can review and monitor CPU usage through specific tools. On Windows, the Performance tab of the Windows Task Manager displays the rate of CPU consumption.

Using AVG Security Toolbar

A recent update by AVG to its AVG Security Toolbar extension caused Firefox to use significantly increased amounts of CPU and memory. This is a bug in the extension that AVG is working on fixing. For the time being, you can disable the extension.

NOTE: It is only the extension that should be disabled. You don't need to remove your anti-virus software.
  1. At the top of the Firefox window, click on the Tools menu and select Add-ons. The Add-ons window will appear.
  2. Select the Extensions panel.
  3. In the Extensions panel, select AVG Security Toolbar.
  4. Click the Disable button.
  5. Click Restart Firefox. Firefox will close, then open with the extension disabled.

Only on specific pages

If high CPU usage occurs only while visiting specific websites, the following areas should be reviewed to identify the common issue.



Some websites, such as YouTube, require a plugin called Flash. The Flash plugin is used to display animated or video content inside the web browser window. This plugin is updated regularly, and some sites require the latest version of the plugin to be used.

  1. Download and install the latest version of the Flash plugin For more information, see Flash.
  2. A Firefox add-on, called Flashblock, allows you to selectively enable and disable Flash content on websites.

Adobe Reader

The Adobe Reader plugin enables the display of PDF documents inside the Firefox browser. The plugin is the browser component of Adobe Acrobat Reader, a free tool for reviewing and printing PDF documents. If the problem seems to be occurring when you access PDF documents:

  • Try updating Adobe Reader. For more information, see Adobe Reader.


JavaScript settings and preferences for interactive web pages is a scripting language that is included in many web pages to enhance the experience with menus, sounds, and other features. In an individual site, extensive use of JavaScript may require significant CPU usage. A Firefox add-on, called NoScript, allows you to selectively disable JavaScript on websites. For more information, see NoScript.

Report the website

Occasionally you may come across a page that simply doesn't work in Firefox. If that happens, please report the problem to Mozilla as follows:

  1. Click the menu button New Fx Menu , click help Help-29 and select Submit Feedback.... The Submit Your Feedback page will load in a new tab.
  2. Click on "Firefox makes me sad".
  3. On the Details page that comes up, briefly describe the problem, and enter the URL of the problematic webpage.
  4. Click the Submit button.

On every website


Extensions for Firefox are developed by a community of thousands of individuals spread around the world. These individuals are responsible for the content of their extensions. Sometimes, CPU usage problems can occur due to varying development standards, incompatibility between extensions, and insufficient support for newer versions of Firefox. For more information, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Hardware acceleration

Disabling cursor and bitmap hardware acceleration on some video cards and drivers can reduce CPU usage when displaying images. For more information, see the documentation that came with your video card.

Windows Compatibility mode

Windows Compatibility mode enables the execution of Windows program in an environment that mirrors earlier versions of the operating system. For example, you can execute Firefox in a Windows XP environment that is running the program as if the operating system was Windows 95.

win-compatibilitymode.png Running Firefox on Windows in Compatibility mode has been known to cause high CPU usage. To stop Windows from running Firefox in Compatibility mode:

  1. Find the firefox.exe file. Typically, this file is located here:
    C:\Program Files\Mozilla Firefox
  2. Right-click the Firefox icon, and select Properties to open the Mozilla Firefox Properties window.
  3. Select the Compatibility tab, and make sure Run this program in compatibility mode for: is deselected.

Windows prefetch

Newer versions of Windows utilize a mechanism to accelerate the boot process. On Windows XP and Vista systems, the operating system must load many individual pieces of files during the boot-up process for the operating system and applications. Windows maintains a record of the pieces that are loaded during the boot process in a folder. Subsequent rebooting can use this trace to identify files that would be more efficient to load entirely at one time, instead of loading individual pieces across multiple reads of the hard disk. This prefetch mechanism can accelerate the overall performance of the computer.

However, the method of deciding which files to load may not be efficient for Firefox and can require significant CPU resources. You may experience faster performance by removing prefetch files on Windows XP or Windows Vista systems. The prefetch files are stored here:

Delete any filename beginning with: FIREFOX

Based on information from Firefox CPU usage (mozillaZine KB)