I think something that would be useful as a dev tool in FF, which only chrome right now has a decent tool for is restful url testing, similar in part to https://chrome.google.com/webstore/detail/advanced-rest-clie... I haven't seen anything even remotely similar for FF, maybe a optional plugin for this kind of tool, it would make my development process much quicker for the client-side code which is being used for restful interfaces. especially if we can maintain our session when submitting information.
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.
