Firefox, Thunderbird and SeaMonkey allow to edit some hidden preferences through a special interface, usually referred  as about:config.

• To access it in Firefox and SeaMonkey enter about:config in the location bar and then Enter in the keyboard or press the Go button.

• To access it in Thunderbird, in the Tools menu, select Options. In the Advanced page, select General and press the Config Editor button.
• To locate a specific preference, you can enter any part of the preference name in the Filter text box. Preferences are filtered as you type.

• To edit an existing preference, right-click on the preference and select Modify from the context menu. If the preference is a boolean type (True or False) then it will say Toggle instead as there are only two possibilities. You can just double click a boolean preference to toggle it.

• To create a new preference, right-click on any other and select New, and a preference type which can be a string (text), an integer (number) or a boolean type (true/false). Enter a name for the preference , and press OK. Enter a value appropriate for the data type (text, number or true/false) and you are ready.