Firebug is brilliant

I've just downloaded the Firebug extension for Firefox. It was always a really useful development tool now it’s just got 100 per cent better. No web developer should do without not that I am a developer but useful just the same.
Firebug lets you view and edit the HTML and CSS of any page.
It tells you exactly which CSS rules affect the styling of the element you select in the style inspector - not only which rules affect it directly but also which rules it inherits properties from. It lets you edit anything and toggle CSS declarations on and off, with the results immediately showing on the page.
Firebug also has a layout inspector that displays a nice graphic of the currently selected element’s box model properties, and a DOM inspector that lists all DOM properties and their values.
There’s even a JavaScript debugger and the Net inspector. A brilliant piece of kit.