FireStorage Plus! A great new FireFox Addon which is officially recognized by Firebug is available from the Mozilla gallery: http://addons.mozilla.org/en-US/firefox/addon/firestorage-plus/
This plugin extends Firebug to give you the functionality you need. As with Firebug itself, FireStorage Plus! is currently under heavy development so new features come out every couple of days...
It is stable and ready for use...