First, tell MozUnit what editor it should start when you want to create or edit test files:

1. Go to the about:config URL
2. Find the extensions.mozlab.mozunit.editor
3. Enter the command that will start the editor
   • Path must be absolute
   • %f will be replaced by the file name
   • %l (optional) will be replaced by the line number
   • %c (optional) will be replaced by the column number
   • For example, the command to use on Unix to start Emacs at the appropriate line and column would be:

     /usr/bin/emacs +%l:%c %f
     

In Firefox, select Tools → MozLab → Open MozUnit Runner from menu.

In the runner, select File → New from menu, and enter a filename when required (e.g. tc_foo.js). The editor will open and it will contain a testcase skeleton you can modify.