gui:config makes it easier to change preferences that can only be found in the “about:configâ€. The extension supports a comfortable way to apply changes to these options. It displays the options, included in the current version, using checkboxes, dropdownlists and textboxes.
https://addons.mozilla.org/en-US/firefox/addon/5523
https://addons.mozilla.org/en-US/firefox/addon/5523