You can see an error message in the Tools > Error Console.
That code now gets a null principal for security reasons and doesn't have any effect (Bug 656433).
You need to create a (keyword) bookmarklet and run the code by invoking that bookmark.
NoScript can allow you to run such code via the location bar, see:
At least add about:config entries to restore the useful features that keep being removed.
Although entering JS through the URL bar has been disabled for very good security reasons, in the latest Firefox release it's possible to do the same but by different means.
If you go to the "Web Developer" menu (or if you're on a Mac, Tools > Web Developer) and select "Scratchpad" up will pop a box where you can enter some JS, right click and "Run" it on the webpage. The result is exactly the same as one would have by entering stuff into the URL bar.