There was a Firefox bug (656433) that removed the ability to run any JavaScript bookmarklet from the URL bar.
You now have to find a workaround, or run it from the Web Console (Web Developer -> Web Console). Make sure to remove the "javascript:" prefix if you run it from the Web Console.
You could also run it as a bookmark: create a new bookmark with the JavaScript statement as the URL. Then just "open" the bookmark on the page that you want to run it on.