This is an extension for advanced preferences. gui:config makes it easier to change preferences that can only be found in the “about
:config”.
https://addons.mozilla.org/en-US/firefox/addon/5523 Gui:config Gives Easy Access to Hidden Firefox Settings
I cant see much thats of use to me with this extension, its a GUI for some of the settings in the about
:config. If you like messing with cache sizes and configuring bookmark backups you wouldnt be the sort of user who would need this anyway.