It's hard to imagine a time before Developer Tools. They make everything easier. Except making a cup of tea.
Since Firefox 4 last March, Mozilla have begun to include their own developer tools built in to the Firefox browser. Whilst Firebug still remains available as a plugin and is still supported by Mozilla, the browser vendor identified the need to include built-in developer tools – possibly so it has more control over future features and targets of developer tools within Firefox.