Firefox 11 Style Editor
The new style editor in Firefox 11 is your playground for doing live editing of CSS changes on your web pages. Got a problem with the CSS in your newly developed Orchard, Umbraco, or Drupal website? Open the page in Firefox 11 and display the inspector and style editor. Pinpoint those troublesome areas using the inspector and then make live changes to the CSS as displayed in the style editor. Once you found the correct CSS change go ahead and save the CSS using the style editor, too. Amazing the quality of tools you can now find just in the browser.
Firefox 11 3D DOM Viewer
I am not exactly sure how to use the new 3D DOM Viewer to my web development advantage yet, but man is it really cool to see your web page's DOM in 3D. This only works if you are WebGL capable. Next to the HTML Button on the Inspector will be a 3D Button. Press that and with the movement of your mouse you can start to see your web page DOM in 3D. Good fun!