Fix: XULRunner Error Platform Version is not Compatible

If you are a Mozilla Firefox user then you might see an error named Xulrunner Error when trying to open Firefox browser. This error will appear out of nowhere and it will prevent you from using the Firefox browser. This error can also appear when turning on the Mozilla ThunderBird as well. In fact, most of the users experienced this issue with both of the applications.

Xulrunner Error

What causes the Xulrunner Error to appear?

Here is what causes this error

  • Incomplete Update: The main reason for this error is an incomplete installation/update of the Firefox browser. Firefox is usually set to automatically install updates and this process can be interrupted because of bad internet or a sudden system shutdown or an antivirus. Whenever this happens, your Firefox update doesn’t install completely and leaves you with an installation with a mixture of old and new updated files in the Firefox program folder.


The solution given below is for the Mozilla Firefox. However, as mentioned in the beginning of the article, this problem can occur for the Mozilla Thunderbird users as well. The reason for seeing this error when trying to open Thunderbird is the same as Mozilla Firefox. So, the solution given below will work for Mozilla Thunderbird as well. You just have to reinstall the Mozilla Thunderbird instead of Mozilla Firefox but the other steps should be same.

Method: Clean Install Firefox

Since the problem is caused by an incomplete update installation, the usual solution is to perform a clean install of the program. This means that you have to uninstall the program and get rid of all of its files so there’s nothing left on the system that might interfere with the new installation. You can then install a fresh copy of the program and that generally solves the problem. So, follow the steps given below to perform a clean install of Firefox.

  1. Hold Windows key and press R
  2. Type appwiz.cpl and press Enter
Type appwiz.cpl and Press Enter To Open Installed Programs Page
  1. Locate Firefox and select it
  2. Click Uninstall and follow the on-screen instructions. Note: Do not remove your personal files when the uninstaller ask you to choose whether to get rid of personal files or not. Your personal files include your Firefox profile, bookmarks, and several other files so make sure you don’t do that.
Select Mozilla Firefox And Click Uninstall
  1. Once the program is uninstalled. Hold Windows key and press E
  2. Type C:\Program Files\ in the address bar and press Enter
  3. Delete your Mozilla Firefox folder
Navigate To Program Files and Program Files (x86) and Delete Mozilla Firefox Folder
  1. Type C:\Program Files (x86)\ in the address bar and press Enter
  2. Delete your Mozilla Firefox folder
  3. Now reboot
  4. Once the system is rebooted, click here and download a fresh copy of Firefox.

Install the latest version of Firefox by running the downloaded file and everything should be fine.


Kevin Arrows

Kevin Arrows is a highly experienced and knowledgeable technology specialist with over a decade of industry experience. He holds a Microsoft Certified Technology Specialist (MCTS) certification and has a deep passion for staying up-to-date on the latest tech developments. Kevin has written extensively on a wide range of tech-related topics, showcasing his expertise and knowledge in areas such as software development, cybersecurity, and cloud computing. His contributions to the tech field have been widely recognized and respected by his peers, and he is highly regarded for his ability to explain complex technical concepts in a clear and concise manner.