From MozillaZine Knowledge Base

userChrome.css in the chrome folder is a CSS file that can be used to change the way Mozilla applications' interfaces look. This file does not exist in a new profile. You can create it manually. If you just want to move/remove/rename menus and menu items or change/disable shortcuts use the Menu Wizard add-on instead.



To create this file manually, 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 userChrome.css. Copy and paste the following line as the first line of the file (if it doesn't already exist). This is called the namespace line and you must not alter it. Anything you add later on should always be below it

@namespace url("");

To create this file automatically, download the .zip file - Paste the .zip file in your profile folder and unzip/extract it there. A chrome folder will be created with the userChrome.css and userContent.css files already within the folder, together with example files of each.


  • If you have more than one profile, then each of your profiles has a separate userChrome.css file.
  • There is another chrome folder where Thunderbird's code is installed. Do not create a userChrome.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 Mini Tutorial (old) see Section 4 here -


For sample styles, see:

Moving and copying

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


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

Related files and folders

See also

External links