Use this extension to "fix" that:
https://addons.mozilla.org/firefox/addon/browse-by-name

Thanks for your reply the-edmeister but it's not working for me--still get a page full of search results when I type "mozilla.com". And when I do get support.mozilla.com open, the URL is nowhere to be seen so I can't copy/paste it, only the original "mozilla.com" that I typed is visible in the search bar.

What has happened is the original URL address bar is now a search bar in 5.01, am I right? So if I'm in the Google home page, I have TWO search bars open. If so, it is such a stupid step backwards. If it ain't broke, fix it seems to be the motto.

Unless I've done something to mess up the Navigation Toolbar and can figure it out, its back to v4 for me. Sigh.

Sorry, the change was made in Firefox 4.0, not 5.0.1. That extension is supposed to change the keyword.URL back to what it was in Firefox 3.6.x and earlier versions. Typing mozilla alone should give you a search, but mozilla.com would "guess" and load http://www.mozilla.com/en-US/firefox/fx/ - that's how it works for me in Firefox 5.0. My suspicion is that something is messed up in your Firefox installation, Try a new Profile and see if the actions you see are as I say - different from what you are now seeing.
http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows