Better auto-complete for css fields for the css editor in the new FF, such that it's on par with the firebug auto-complete functionality in the dom/css inspecter
Auto-complete for css properties and easier value editing (up/down arrows) would definitely be a huge help. With that, I could see myself switching to FF for CSS development.
The responsive features in the web console - which imho HAS to be monosopace - are also definitely exciting, and make me want to switch to FF.