Firecookie gives you the power to manage your cookies on Firebug from within your browser. You can see which cookies you currently have on your browser, create a new one, delete ones that are on there, see the cookies that are enabled for the site that you are visiting, and even determine which permissions you allow for the cookies, among other things.
Firecookie comes with Firebug 1.10, which means that you won’t have to install it if that is the version of the browser that you have. Should you discover any bugs in the add-on, which is quite possible because it is still in beta, you can report them at http://code.google.com/p/fbug/issues/list.