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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.

Note: If you send performance data, Mozilla will gather data including the CPU usage for your Firefox, which will help making Firefox better for future versions.

Updating to the latest version

The latest Firefox version includes improvements about CPU usage. Update to the latest version.

Extensions and themes

Disabling CPU consuming extensions and themes

Extensions and themes can cause Firefox to use more CPU than it normally would.

To determine if an extension or theme is causing Firefox to use too much CPU, start Firefox in its Safe Mode and observing its CPU usage. In Safe Mode, extensions and themes are disabled, so if you notice a significant improvement, you can try disabling or uninstalling extensions.

For more information on starting Firefox in Safe Mode and on how to find which extension or theme is causing your problem, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems.

Hiding intrusive content

Many web pages have content you don't need, such as ads, but which still use CPU resources to display its contents (see the plugin section below). Some extensions allow you to hide bothering content:

Adblock Plus allows you to hide ads on websites.
Flashblock allows you to selectively enable and disable Flash content on websites.
NoScript allows you to selectively enable and disable all scripts running on websites.

Plugins

Plugins to display special types of content can consume large amounts of CPU, especially older versions.

Updating your plugins

To check to see if you have the latest versions of all your plugins, go to our Plugin Check page.

Disabling CPU consuming plugins

You can test to see whether one of your plugins is causing Firefox to use too much CPU by selectively disabling some of them:

At the top of the Firefox window, click on the Firefox buttonOn the menu bar, click on the Tools menuAt the top of the Firefox window, click on the Tools menu, and then click Add-ons. The Add-ons Manager tab will open. Click the menu button and choose Add-ons. The Add-ons Manager tab will open.
In the Add-ons Manager tab, select the Plugins panel.
Click on a plugin in the list to select it, then press Disable to disable it.
Repeat for some of the plugins in your list.

At the top of the Firefox window, click on the Firefox buttonOn the menu bar, click on the Tools menuAt the top of the Firefox window, click on the Tools menu, and then click Add-ons. The Add-ons Manager tab will open. Click the menu button and choose Add-ons. The Add-ons Manager tab will open.
In the Add-ons Manager tab, select the Plugins panel.
Click on a plugin in the list to select it, then select Never Activate to disable it.
Repeat for some of the plugins in your list.

After disabling some of your plugins, close and restart Firefox, and observe its CPU usage. If you don't see an improvement, you can enable those plugins again and try with a different set.

If you do see an improvement in Firefox's CPU usage after disabling a certain plugin, you may leave it disabled. If its use is widespread on Internet, try to find an alternative lighter plugin.

For PDF readers lighter than Adobe Reader, see View PDF files in Firefox without downloading them.

Checking Flash hardware acceleration

For certain plugins that play videos such as Flash, the content rendering in full screen can be accelerated by hardware. It eases CPU usage.

Navigate to a page that shows a Flash video.
Right-clickHold down the Ctrl key while you click on the video player and click Settings… in the context menu. The Adobe Flash Player Settings screen will open.
Click on the icon at the bottom-left of the Adobe Flash Player Settings window to open the Display panel.
Check Enable hardware acceleration is selected.
Click Close to close the Adobe Flash Player Settings Window.

Checking Firefox hardware acceleration

Firefox hardware acceleration eases CPU usage.

Check hardware acceleration is turned on and your graphic drivers are up-to-date.

Based on information from Firefox CPU usage (mozillaZine KB)

Firefox