Right, the Firefox error you get makes sense, since we don't *require* HTTP auth while you are logged in to the site (via sessions/cookies). You can download 'tip.gz' without providing your username and password at that point.
Now with regards to automated fetching, you *must* use Digest, as we do not support basic. The challenge should be picked up properly, even mercurial does this correctly (when using hg on the command line.) If you have problems using curl/wget, I'd like a full verbose traceback of how they fail.
You can troubleshooting your pages status code and response using this <a href="http://soft-net.net/SendHTTPTool.aspx">
Free http status code tool </a>
It free and it give you alot of information about the response from the page.