Addons » Web DevelopmentCool utility for cleaning up a page (for printing or better viewing) as well as for web development. Displays a red rectangle on the element under the mouse -- hit "w" to go wider (get the containing element), then hit other keystrokes to remove, etc

See also
FireXPathFireXPath is a Firebug extension that adds a development tool to edit, inspect and generate XPath expressions.
FirediffTrack Changes in Firebug
TortoiseSVN MenuAdd TortoiseSVN commands to link context menus to allow easily browsing Subversion repositories. Must have TortoiseSVN already installed.
FirecookieFirecookie is an extension for Firebug that makes possible to view and manage cookies in your browser
Library DetectorDetects what javascript libraries are being used on the current page and displays the result as icons with detailed tooltips in the statusbar.