Like Most DLL Errors, XUL.dll Is Missing Or Corrupted

Windows has a long history of DLL errors, with many programs ceasing to load up due to either a dll file being missing, corrupted or incompatible with your system. The main problem for the majority of users is that identifying the underlying cause of this problem can be difficult.

If you don't know, DLL files are essentially mini applications which interface with base levels of the Windows OS and allow higher-level (GUI) applications to communicate with your system.

DLL files are called Dynamic Link Libraries because they're meant to deliver library-level functionality to a number of different applications on the system. To this end, most DLL files are stored in the C:/Windows/System32 folder, but can be placed anywhere on the system (they're all stored in the registry of your PC). *Nix systems also use DLL files, except they have an SO extension.

In the case of XUL.dll, it's used by a number of applications which have been built on top of its platform. It stands for XML User interface Language, which is a platform designed by FireFox to create cross-platform applications. As I've never developed using XUL, I cannot begin to describe its core functionality. I can, however, give a definition of where it sits in the Windows system...

  • Name: XML User interface Language
  • Location: C:\Program Files\x\xulrunner\xul.dll
  • Version: Depends

The core reason your system will showcase XUL.dll errors is because it either has an incorrect version of the file, or the file is missing entirely from your system. The steps to resolve the issue include ensuring the broken application is installed correctly, as well as ensuring your Windows installation has the correct version of the file.

Step 1 - Reinstall Firefox (or Damaged Application)

Uninstall FireFox

The solution to xul.dll errors is much the same as other DLL errors, namely to reinstall the offending application as well as then address any potential issues with the DLL file or the way in which Windows has registered it.


To reinstall an application, you need to load up the Windows "Add / Remove Programs" dialogue, which is slightly different depending on which version of Windows you're using.

If you're seeing the error when using a particular application, it suggests that the program is trying to access xul.dll and is unable to either find it, or find one that's compatible. The best way to resolve this is to re-install the application, in the hope that it will place a new version of the DLL onto your system.

To do this, you can follow these steps:

Windows 7

  1. Click onto "Start" (bottom left of screen)
  2. Select "Control Panel"
  3. Select "Uninstall a Program" under the "Programs" subheader
  4. From the list, select the application causing problems
  5. Right-click and select "Uninstall"

Windows 10

  1. Right click on the "start" button (bottom left)
  2. Select "Apps and Features"
  3. Scroll down the list
  4. Select the application causing problems
  5. Right-click and select "Uninstall"

After removing the application, it's advised that you should restart your system.

This will clean up any of the processes associated with installation of programs. After the system loads up again, you'll be able to reinstall the program. The best way to achieve this is to download a fresh copy of the app from the Internet and then let it install. It may be the case that an updated version has been released in the interim.

2. Replace XUL.dll

Download New XUL.dll File

The next step is to replace XUL.dll on your system.

Whilst this can be done by manually replacing the XUL.dll file directly, it's much better to re-install any application which may be using it. This way, you'll know you have the latest and most reliable version of the file installed.

Unfortunately, since Mozilla ceased including XULRunner (which provides XUL.dll) as a standalone build in 2016, you may need to install the independent version of the technology to get it working on your system. There are two things to do - download a new version of XULRunner and download a new version of XUL.dll directly...


XULRunner has been extracted to its own repository. It is available at SoftPedia - if you search for "XULRunner download softpedia" online, you'll be able to download the latest version of the software.

Once downloaded, install it onto your system and then restart your PC. If the installation was successful, Firefox (or whichever application was showing errors) should recognize the new XUL.dll file placed onto the system, allowing you to run the app.

If the application doesn't work even after XULRunner has been installed, you will need to remove XULRunner as it may conflict with other applications on your system. 


If the installation of XULRunner doesn't resolve the problem, you'll need to download a fresh copy of XUL.dll from the Internet and install it into your Windows system (re-registering it as required).

The steps to do this are simple (there are a number of DLL repository websites available to help):

  • Look for "xul.dll" download online
  • From the results, find any which give a direct link to a file
  • Once you find a website which has a file, you'll want to download it to your system
  • Click into the ZIP and then extract the DLL to your hard drive
  • From here, right-click on it and select "Copy"
  • Browse to C:/Windows/System32
  • Right-click and select "Paste"
  • If it prompts you to overwrite an existing file, click "Yes"

This will place a new copy of xul.dll on the system.

From here, you'll want to ensure it is completely registered in the Windows registry. To do this, you need to load up a new instance of CMD (Command Prompt) and then run Regsvr32:

  • Press Windows Key + R on your keyboard (brings up "Run" dialogue)
  • Into the text box, type CMD and press Ok
  • When CMD appears, type regsvr32 /u xul.dll & press Enter
  • After this, type regsvr32 /i xul.dll and press Enter

Upon typing both commands into the CMD, restart your PC and test the application to see if it works.

3. Update Firefox

Install FireFox

If you're experiencing the error with Firefox (as most people do), you'll want to ensure you are running the most up to date version of the application.

The best way to do this is to download a fresh installer from the Internet and then let it install for you. Apart from the Firefox website itself, you should be able to do this by heading to CNET or Softpedia, both of which provide up to date repositories for the Firefox, and Thunderbird. 

After downloading a new installer for the system, you should place it on your hard drive. the trick after doing this is to then browse to the Firefox installation folder (C:/Program Files/Mozilla Firefox) and delete it. This will have to be done after closing any instances of Firefox running on your system.

Deleting the folder won't do anything in itself, but will ensure that any new files put into the system will be brand new (rather than just copying files that are not already present on the system). After deleting the folder, you should then double-click on the installer you downloaded. This will place all new files onto your system, which should ensure that XUL.dll is appropriately replaced.

4. Clean Out Viruses / Malware

MalwareBytes Scan

Finally, you need to clean up any potential malware your system may have.

A common problem for web centric applications (such as Firefox) is they will be targeted by a huge number of malicious applications. These applications are colloquially known as "malware" (malicious software). 

In the case of Firefox, it could be the case that one of its components has been targeted by a rouge malware infection. To ensure this is not a problem, you need to scan your system with both anti-malware and anti-virus solutions:

  • Open up the antivirus application running on your system
  • If you are not running one, you should have Windows Defender running
  • This, along with any other antivirus tool, will be available in the bottom right corner of the task bar
  • Let it perform a FULL system scan
  • This may take several minutes - after doing it, restart your PC


  • After the restart, download a program called "MalwareBytes"
  • This is a free tool which you can download from their website - it cleans out malicious software and is a great way to ensure you don't have hidden infections on the system
  • Once you've downloaded MB, install and run it
  • Let it perform a FULL system scan
  • Once complete (it may take several hours), restart your system

If no viruses or infections are found, you should try Firefox again. 

If infections were discovered, you should look to re-install Firefox as to resolve any potential problems that may have resulted from the virus infection.


If you still are experiencing issues with XUL.dll, you may wish to look at trying to receive direct support from an expert who has experience resolving this type of error.

There are a number of communities who have a strong community of experts, the most appropriate for this error probably being SuperUser. If you sign up to that, or a number of others, you'll be able to ask about how to fix the error and someone should provide more specific advice on how to resolve.

The big problem with online articles is that although they work well to provide users with a general synopsis of what to do, they are quite scant on providing specific support to actual problems. You may have conflicting software, or some other issue preventing your computer from being able to access the XUL library properly. As such, it may be better to seek further - specific - support from a respective online community.