YES, with the latest release of firefox one of the best update is the Firefox Firebug which is inbuilt. While Chrome had it right from the start, Firefox finally decided that this will be a feature out of the box.
The new interface with dark theme is definitely much better than the current firebug theme and there are quite a few nice improvements over the current Firebug interface.
The dark theme gives a nice modal effect with the element nicely highlighted. Also the tag is displayed which is very handy when scanning through the elements and when we need to study the structure of HTML in any part of the website.
Another feature which impressed me a lot is the two sections “HTML” and “Style” which are neatly designed. We can now decide if we require both or just the CSS and switch off the other section.
The Full width for the HTML is for sure a life saver. I very well remember sections of my site where the HTML structure is so deep that I have to scroll horizontally to see the HTML markup. This UI is a life saver. And it complements the Style tab which should have been vertically long right from the start. CSS styles are such that we need vertical scroll.
All in all the new feature is definitely a great improvement to the Firefox list of features. For the complete list of features that are updated in the new version check this link.