Setting up the prefs.js File
Before this file can be setup it is essential to know what a preference is.
A preference is any value or defined behavior that can be set (presumably, one setting is preferable to another). Preference changes through the user interface take effect immediately, and are automatically saved to prefs.js in the user profile. These remain persistent between application sessions.
All preferences files may call pref() and user_pref() , while the config file in addition may call lock_pref().
To protect privacy by preventing inadvertent loading of a preferences file in the browser, the first line of the file is made un-parseable and skipped on loading.
The prefs.js file tells XULRunner which XUL file contains the main window.