Why is Firefox running slowly?

Firefox running slowly
Go slow: find out what's festering on Firefox

The Firefox browser is a lot more nimble than it used to be, but sometimes the fox starts running more like a badger with a breeze block strapped to its back. Here’s how to find out what’s making Firefox run slowly on your computer and how you can remedy the situation.

What causes Firefox to run slowly?

First, you need to find out what’s bringing the browser to its knees. Likely culprits are tabs running high-definition video or demanding web applications such as the Twitter client TweetDeck. These can put a lot of strain on the computer’s processor or memory, causing the browser and the entire computer to run more slowly.

Another likely cause of browser slowdown is ads opening in the background, especially video ads. News sites are particularly bad for this. You’ll read a story on a newspaper’s website, leave the tab running in the background, and two minutes later it will start automatically playing a video ad. If you’ve muted your computer, these can be hard to spot. The first thing you notice is when the browser starts crawling.

Tab crashes can also cripple the browser. A website can cause a tab to freeze or consume tons of memory. Until you identify and kill the culprit, it can bog the browser down. Thankfully, Firefox has a built-in Task Manager that helps to identify any of the above.

Find out what’s making Firefox crawl

To identify why Firefox is running slowly, enter the following term into the browser’s address bar (note, there’s no space after the colon):


This will bring up Firefox’s Task Manager, which behaves in a similar way to the Windows Task Manager. It will look something like this:

Firefox task manager

As you can see, it lists all the open tabs and add-ons running on your machine and reveals how much ‘energy’ and memory they are consuming. Anything that’s marked ‘high’ for a sustained period under the Energy Impact heading is likely to be a problem. Likewise, any tab that is swallowing much more memory than the others.

Sometimes, it’s possible to drill down in further detail about what the problem is with a particular site. Here, for example, I’m visiting the Daily Mail website, a site notorious for battering visitors with ads, auto-playing videos and other performance-sapping add-ons.

If I click on the little arrow next to the Daily Mail’s listing, it expands to reveal the items on the site that are affecting performance:

Firefox running slowly

In this case, it’s the trackers that the Daily Mail website employs on behalf of advertising partners (they’re marked with the red footstep icons).

If a particular tab is using more than its fair share of energy or memory, you can instantly close it from within the Task Manager by clicking on its name and clicking the X that appears on the right-hand side of the window.

Note that any unsaved work (if you’re writing in WordPress, for example) or data entered into a form in that tab will be lost.

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About the author

Barry Collins

Barry has scribbled about tech for almost 20 years for The Sunday Times, PC Pro, WebUser, Which? and many others. He was once Deputy Editor of Mail Online and remains in therapy to this day. Email Barry at

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