It's one of those things that's really annoying and the solution is not very easy to find because searching Google for www and .com doesn't give you anything useful.

1. In the address bar type: about:config.
2. Accept the warnings about pending doom.
3. Enter: browser.fixup in the search box.
4. Double clicking browser.fixup.alternate.enabled will disable it completely.
5. If you want, you can keep the addition of www and .com separate buy double clicking either browser.fixup.alternate.prefix or browser.fixup.alternate.suffix.
6. The changes will work immediately.

You may find that your address bar is trying to be like the Google address bar, which seems a bit pointless as there is a whole search box for exactly this purpose. If you are finding that every time you get an address wrong it sends you over to Google then repeat the above steps but change keyword.enabled to false.

Firefox has always been a favourite of developers because Firebug support was so good. That's all changed now.