Step-by-step guide on how to fix XULRunner error in Firefox

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Many of our readers have complained about the Firefox XULRunner error when reinstalling or updating the browser. This can happen if you have old files in your Firefox program folder, bad internet, or security software interfering with the Firefox installer.

First, make sure you have a stable internet connection, then follow the steps below one by one to resolve the issue.

How to Fix XULRunner Error in Firefox?

1. Back up your Firefox data

Launch Firefox, click the menu button, and then select Bookmarks.
Click Manage bookmarks.
Click Import and backup and choose Export bookmarks to HTML from the context menu.
Now save the file to your chosen location.

2. Temporarily disable security software

Here we show you the steps using Avast security software. Locate the system tray in the lower right corner and right-click on the Avast icon.
From the context menu, select Avast Shields Control.
You will get options such as: Turn off for 10 minutes, Turn off for 1 hour, Turn off until computer restarts AND Turn off until manually turned on. Select any option to disable.
Click Yes when prompted.

In other security software, look for similar options such as Real-time Protection, Real-time Scanning, or Virus Protection.

Disabling your antivirus software can also fix other Firefox issues, such as: Firefox does not works on Windows 10. To learn more, check out this guide.

3. Uninstall Firefox

Press Windows + R open Start window.
Type applicationwiz.cpl and click APPROX open Programs and functions window.
find Mozilla Firefox from the list of installed programs.
Select and click Uninstall.
On Uninstalling Mozilla Firefox window, click Next.
Follow the on-screen instructions to complete the process.

4. Delete Firefox installation files from the Applications folder

Press Windows + to me open File Browser.
Navigate to this path depending on your system: C:\Program Files Or C:\Program Files (x86)
Don’t forget to replace the system drive letter with C.
Find the Mozilla Firefox folder, select it and click Remove Icon.
Press Windows + R open Start window.
Type %local application data% and click APPROX open Application Data\Local file.
Look for any folders related to Firefox. If you find any, select them and click Remove Icon.
Restart your computer.

5. Reinstall Firefox

Go to the Mozilla Firefox download page.
Click download now and the installer will be downloaded to your computer.
Double-click the installer file and follow the on-screen instructions to complete the process.

Reinstalling your browser may help resolve other issues, such as installation being blocked; read this guide to learn more about this issue.

Once you have reinstalled Firefox, you can reverse engineer Step 2 to enable your security software, then launch Firefox and import your bookmarks.

Remember to always delete the associated folders from the programs folder after uninstalling the application to avoid such problems in the future. Also, make sure that your browser is always up to date.

If your Firefox browser is running slowly, you can follow these simple tips to speed it up. You can read all the details here.

If you have any suggestions or comments on this topic, you can post them in the comments section below.

Srishti Sisodia

Windows Software Expert

Srishti Sisodia is an electronics engineer and a writer with a passion for technology. She has a vast experience in exploring the latest technological advancements and sharing her insights through informative blogs. Her diverse interests bring a unique perspective to her work and she approaches everything with dedication, enthusiasm and a desire to learn. That is why she is a part of the Windows Report review team, always eager to share her real-life experience of any software or hardware product. She also specializes in Azure, cloud computing and AI.