That new Inspector thing is really annoying. I always accidentally open it instead of Firebug. From a developer stand point, Firebug stills beat all other browser dev-tools by a mile and is the main reason I still have Firefox on my dashboard.
You can turn off the built-in Inspector by going to about:config. There's an option in there (I forget the name of it off the top of my head... probably something like "inspector"!) and you can change the Boolean value to 0 and it will no longer appear in the context menu.
