From MozillaZine Knowledge Base

userContent.css in the chrome folder is a CSS file that you can use to change the way web sites and e-mails look. This file does not exist in a new profile. You can create it manually.

Note:  This file only affects your Mozilla application's display (when you are using a particular profile). It does not actually change any web page or e-mail message. For example, if you use the file to change colors in e-mail messages, it does not affect messages that you send—so people who receive messages from you will see them using their own choice of colors, not using your choice of colors.



To create this file, use your operating system tools.

Go to your profile folder. Then go to the chrome folder there, creating it if necessary. In the chrome folder, create a plain text file named userContent.css.


  • If you have more than one profile, then each of your profiles has a separate userContent.css file.
  • There is another chrome folder where Thunderbird's code is installed. Do not create a userContent.css file there—it will not work.


This file is designed to be manually edited using a text editor. Anything valid in a CSS file is valid in this file. To make the changes take effect, you must restart the application.

For sample styles, see:

Moving and copying

You can move or copy userContent.css to a different profile. If it imports or references other files such as images, then you must also move or copy these other files. To make the changes take effect, you must restart the application.


You can delete userContent.css to get rid of any user styles defined there. To make the change take effect, you must restart the application.

Related files and folders


For some information about common problems, see: Editing configuration – Troubleshooting

External links

  • The ChromEdit extension provides a convenient way of editing userContent.css and other configuration files.
  • The Stylish extension provides another way of changing styles in web pages and e-mails. It does not use the userContent.css file.
  • The GreaseMonkey extension provides a way to run scripts in web pages.