This question was asked the other day on an IRC channel I was in: How do you make changes to about:config without opening Firefox. Though this may seem silly at first, there are quite a few reasons why this would be practical and necessary, such as wanting to deploy Firefox on multiple machines with the same about:config settings. Luckily, it is quite easy to set, but you may want to have Firefox in front of you while you do it.

For those of you who do not know, about:config is a way to modify more advanced features of Firefox. You probably do not want to mess with prefs.js unless you know what you are doing. You can learn more about about:config here.

Before continuing, close down all Firefox processes.

1. Navigate to your user profile and open prefs.js in a text editor.

2. Scroll to the bottom of the document.

3. Now enter your about:config hack in text form. For example, the about:config hack in this article would be represented as:

user_pref("browser.tabs.closeButtons", 1);

user_pref(" ", ); is the basic formula that is used. In the quotes you place the name of the preference as it appears in about:config. after the comma you place the configuration option, such as a number or true or false.

Now you can save the document. When Firefox is reopened with that profile, the settings will be applied. You can modify them further in about:config again.