The Web Developer extension adds various web developer tools to the browser.
View HTTP headers of a page and while browsing.
Cookies manager to view, edit and create new cookies. It also shows extra information about cookies, allows edit multiple cookies at once and backup/restore them.
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
Firecookie is an extension for Firebug that makes possible to view and manage cookies in your browser
Firecookie is part of Firebug since Firebug 1.10