Doesn't work for me, but data:text/html,<pre contenteditable>Hello</pre> works for me.
(If that still doesn't work, out of curiosity, what browser are you using?)
data:text/html,<pre contenteditable/>
What's happening is in fact a little bit more complex than that. Both browsers create the pre element (you can verify that by typing that URI and hitting the tab key to switch focus from the address bar into it).
Now, where Chrome renders a line-sized pre element even when it's empty, Firefox renders it as a 0px element until you feed text into it (that's probably its standard behaviour for pre elements without a child text node, but I can't be bothered verifying that).
Boy, I love getting a rush out of figuring out something completely irrelevant! :-)