The next major Firefox update will feature a new tool aimed for web developers called Web Inspector.
In its current state it is a relatively simple utility (accessible from the Tools menu) that allows to navigate the current web page structure (DOM, document object model) to easily identify what element is being displayed and help web design debugging.
However the goal is for a more powerful tool than that according to Mozilla Wiki, could include rulers, a style viewer, palettes, properties browser, and not much more. Simplicity is a goal and it is not an attempt to replace Firebug, a full suite of powerful web development aids. You can see a mockup from earlier this year below.
If it looks and sounds familiar to you it is because Firefox used to come with the DOM Inspector, which provided similar functionality but was removed during Firefox 3 development and made available as an independent extension available from Mozilla Add-ons as it was considered too technical for the vast majority of users.
What has changed? According to Rob Cambell, the Mozilla developer leading the Web Inspector development, “[a] lot can change in a few years. Now every browser ships with a set of development tools. People can still install DOMi or Firebug if they prefer those tools. We didn’t feel that DOM Inspector was the right tool to include in the browser, however.”, he noted in reply to a comment in his blog post. “This isn’t a reversal of that decision as much as it’s a decision to do something different.”, Rob added.