The new Firefox 10 release has a new HTML+CSS inspector included (Ctrl+Shift+I). It's not as feature-filled as Firebug or Dragonfly, but it does the job nicely and looks great. There's also a javascript scratchpad for doing JS testing, if you don't want to install the Firebug addon. (Shift-F4, Ctrl+Shift+K)

Also, if you haven't tried it yet, you should definitely check out the new Tilt 3D addon, which does a 3D visualisation of the webpage.