Are you using Greasemonkey, Scriptish, or another add-on to inject the scripts, or are you trying to do it in another manner, such as using bookmarklets?

Firefox did change how the urlbar works: in the old days, you could paste and run a script from the address bar, but that has been disabled. Instead, you can use a bookmarklet or (if you can give up some screen real estate) the Web Console.