I still use Firebug, mainly because I'm more familiar with it than I am with the Chrome developer tools, I like the UI a lot more, but that's probably my own personal taste rather than it being a better UI. I rate it as being on-par with the Chrome developer tools, and quite frankly I don't consider either to be better than the other for general front-end dev.

That being said, I am really not a fan of the in-built Firefox dev tools. For the life of me I don't understand why Mozilla are dedicating their time to building up their own dev tools when Firebug is the preferred environment for front-end devs that use Firefox. Again, this could just be down to personal taste, but it feels like it's a wasted effort. I'd prefer to see those developers spending time on improving Firebug.