Firefox and Thunderbird both contain an about:config file where you can make changes to your preferences by virtue of a simple right-click. I can't emphasize enough that making changes here may have grave consequences. If you don't know what you are doing, please read all the documentation before proceeding. For a good explanation of what all these about:config entries mean and how they function in the context of both products, go to http://kb.mozillazine.org/About:config_Entries.
Another thing to be aware of is that there may be preferences listed in about:config that are actually Mozilla legacy preferences, so everything in the about:config list may not necessarily apply to Firefox or Thunderbird (as a reminder, Firefox and Thunderbird were both built from Mozilla, so they share the same roots).
Double-clicking any default Boolean entry changes it from "default" to "user-set" as well as changing the value from "true" to "false" or "false" to "true," depending on what has been set. Double-clicking any integer or string value launches a dialog box that allows you to change the value or string. All these changes are written to the prefs.js file.
To access about:config in Firefox, simply type about:config in the URL bar.
Accessing about:config in Thunderbird is a little harder, but there is an extension you can install to access the settings. (You can get this extension from https://addons.update.mozilla.org/extensions/. It is discussed in Chapter 13, "Customizing the Look and Feel of Mozilla Thunderbird," in the "Configuration" section.) Again, I caution you to make sure that you know what you are doing before you make any changes to your mail client.
When the about:config screen launches, you can either add new preferences or edit existing preferences.
Adding a New Preference
Type about:config in the URL bar of Firefox. In Thunderbird, you will need to access about:config by using the AboutConfig extension. Figure E-1 shows the about:config screen.
Figure E-1. about:config in Firefox.
Next, right-click to expose the context menu. As shown in Figure E-2, after selecting New, you will need to choose the type of preference you wish to create. The choices are String, Integer, and Boolean, Strings are textual values, integers are numerical values, and Boolean values are either "true" or "false."
Figure E-2. Creating a new string using the about:config context menu.
After you make your selection, a dialog box should pop up, as shown in Figure E-3. Type the name of the preference in the first dialog box and the value in the second dialog box.
Figure E-3. New string value dialog box.
In most instances, you need to restart Firefox or Thunderbird for the changes to take effect.
Modifying an Existing Preference
To modify an existing preference in about:config, follow the same steps as listed previously, but select Modify from the context menu.
Where to Go for Some Hacking Ideas
The best place to go to hack about:config and user.js is http://www.mozilla.org/support/firefox/tips.