How to resolve issue with cubeb-shm cache files generated by Mozilla Firefox

Case #

You are having profile bloat issues in your EUC environment (such as in Citrix Virtual Apps and Desktops) or you are coming across various issues related to Mozilla Firefox. Issues are occurring when/after viewing video inside Firefox.

Solution #

Temporary fix #

Open Firefox and browse to


Then search for user preference variable called media.cubeb.sandbox.

This boolean variable must be set to false and must be locked so that end-users cannot change it.

You can click on the toggle button on the right to temporarily disable the parameter but for a persistent solution you need to make use of the Mozilla Autoconfig customization tool, since this cannot be done via Mozilla Firefox custom admx GPO templates:

AutoConfig files can be used to set and lock preferences that are not covered by group policy on Windows or the policies.json for Mac and Linux. This method can be used to automatically change user preferences or prevent the end user from modifying specific preferences by locking them.

By using the Mozilla Autoconfig tool, you need to set the media.cubeb.sandbox parameter to false with status locked.

Follow the steps below:

  1. Create an autoconfig.js file under the "C:\Program Files\Mozilla Firefox\defaults\pref" folder and add the following contents to the file.


The autoconfig.js file must use Unix end-of-line (LF), even on Windows systems with Firefox 60 or higher. If you have used notepad to create the file, you need to use Notepad++ to do the conversion from CRLF to Unix LF only. In Notepad++, while you have the file open, go to the Edit menu, select the "EOL Conversion" submenu, and from the options that come up select "UNIX/OSX Format". The next time you save the file, its line endings will be saved with UNIX-style line endings.

2. Create a firefox.cfg file under the "C:\Program Files\Mozilla Firefox" folder and add the following contents to the file.

The solution should be applied the next time each user restarts Firefox in their session.

Permanent fix #

As per, a permanent fix is expected in the upcoming release of Mozilla Firefox, hopefully soon.

You can find more detailed guidance and step-by-step procedures for analyzing and troubleshooting Citrix Virtual Apps and Desktops organized by category in my Citrix Virtual Apps and Desktops Troubleshooting e-book.

Sources #

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