This has bugged me for a long time but it just took a little searching to fix. I enter the URL http://copper/ and Firefox can't connect so it decides that I really meant http://www.copper.com/. This is never what I meant. I don't know who runs copper.com, but they can rest assured that they'll stop getting requests from me for the internal web services that I run at home. The same thing happens even if you use the name localhost. I fully expect that most visitors at http://www.localhost.com/ are Firefox users who are trying to access a web server on their own computer.
Apparently it was turned in to a preferences setting years ago as a result of this bug. The feature is called "domain name guessing" or a URL fixup and according to the site that tipped me off to this it might be a feature of Thunderbird as well.
In current versions (Firefox 2) it can be found and turned off by entering about:config in the address bar. This brings up a huge list of settings. Find the setting browser.fixup.alternate.enabled (typing the name in the Filter box at the top makes it easy to find). Double click the name of the setting or its value to set it to True or False. You want False to stop this behaviour.