Firebug has pioneered the in-browser development tools category. It is a robust, solid debugging tool for web developers.
Firebug also sports the handy DOM explorer and network monitoring tools, but one of its strongest points today is its extensions, which provide extra capabilities. Some popular Firebug extensions include Flashbug, YSlow, Firecookie and FireQuery - A nice list is maintained on the Firebug Wiki.
Firebug's primary competitor nowadays is the built-in Webkit Inspector, provided with Chrome and Safari. The two basically offer similar features, but whereas Firebug's strength is its extensibility, Webkit usually supports new & breakthrough technologies better, such as localStorage, appCache and more.
So which tool do you fancy most? Tell us in the comments section bellow.