The next major Firefox update, Firefox 4 (Firefox 3.7 is out of bounds) will feature an in-built web developers tool called Web Inspector. Though in its early stages this utility can help you to closely inspect elements that are being displayed on the current web page. Analogous to DOM inspector or Firebug inspector, in-built Inspector will support rulers & guides, palettes, layers, a graphical highlighter, it will be fast and pretty but its certainly not a return of DOMi into Firefox says Rob Cambell, the Mozilla developer leading the Inspector development.
Modern web browser do ship with a set of development tools, Opera web browser fox example has a fully featured debugging environment, Dragonfly. Mozilla had Dom Inspector that was removed during Firefox 3 development, its now a (DOM Inspector) plug-in and prefers to ship Firefox browser with “a simple and fun-to-use web page inspector.”
To test this upcoming feature download recently released Firefox nightly for Windows, Linux (32 and 64 bit) and Mac.