I wanted to turn this off, but without turning off other auto complete functionality:

• I want to retain the autocomplete in forms (which can be managed from Tools -> Options -> Privacy)
• I also want to retain the autocomplete in the search box on the right (which can be turned off by right-clicking inside the box, and then checking off "Show Suggestions")

But the Location Bar is a somewhat more involved beast. After hunting in the innards of "about:config" I discovered that this was possible. Just follow these steps:

1. In the location bar, type about:config. The location bar is of course the place where you type URLs. Note that this is your internal Firefox configuration. Don't mess with it.
2. In the text box that appears at the top of this page, enter browser.urlbar.maxRichResults as the preference name. (Tip: copy it from here and paste it into that box.)
3. Set the value to 0 if you wish to disable the auto-complete altogether. I have it set to 2 so I get some suggestions but it doesn't crowd up the experience.