From MozillaZine Knowledge Base
Mozilla applications are highly customizable, and there are a number of ways to change their appearance and behavior, such as through extensions and themes. Basic settings can be changed using the application's own dialog boxes. The main Options or Preferences dialogs can be reached from the "Tools" or "Edit" menus (see menu differences in Windows, Linux, and Mac).
This article describes more advanced methods of customizing Mozilla applications, by editing the configuration files.
Preference settings that are modified from default values are saved in the prefs.js file. Instead of directly editing this file, Mozilla applications include a configuration editor that can be accessed from the application interface.
To modify a preference in a browser application such as Firefox or SeaMonkey, type about:config into the Location Bar (address bar) and press Enter. In Thunderbird, choose Tools –> Options –> Advanced –> General and press the Config Editor... button.
You will be brought to a long list of preferences. To change an existing preference, double click on it. To add a new preference, context click (right-click on most systems) anywhere and choose New and the type of preference. You will be prompted for the preference name and initial value. For more detailed information, see the about:config article.
While it's easier to use about:config for a single profile, it may be easier to use a user.js file if you need to make the same changes in many profiles (see the linked article for more information).
pref set <preference name> <value>
To reset a preference to its default value, use command line
pref reset <preference name>
To show current value of a preference, use command line
pref show <preference name>
How to edit configuration files
The ChromEdit Plus extension provides a convenient way of editing your configuration files. ChromEdit Plus is an unofficial, updated version of the original ChromEdit extension , with more features.
If instead you choose to edit your configuration files using your own text editor, you should consult the Manual editing advice.
If you experience problems when editing configuration files, consult the Troubleshooting article.