In a Land Before Dev Tools | Amber’s Website
A great little history lesson from Amber—ah, Firebug!
Link tags: firebug
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FireScope
This addition to Firebug is rather excellent: a built in reference for whatever you're inspecting.
YSlow for Firebug
The YUI folks have released an add-on for Firebug that will analyse your pages and suggest ways of speeding it up.