I think the about:config warning even comes up in Firefox by default when you first run it as a warning that you can mess things up kind of easily and break the browser.

Things are way more open on N900 than most every other device out there, but there are still many key parts that are closed which totally sucks. The browser is very open though. You can grab the full source code to it if you want and do whatever you like with it.

You are right though about the browser. It's not a place to enter a URL, it's a google search. I don't see how to disable it either. I checked about:config and didn't see anything (could have missed it) and didn't see a plugin/extension for it either.

Perhaps it's due for a rebuild with those things disabled.