Basic Browser Authentication

The easiest and most elegant way to handle basic browser authentication is to supply the username and password in the URL, bypassing the dialog altogether.

require 'watir'
b = Watir::Browser.start 'http://admin:password@yourwebsite.com'

Proxy Authentication

If the above method doesn’t work, eg. you are authenticating against a NTLM proxy, you may have find using the AutoAuth Firefox extension will enable you to automatically dismiss this dialog with your credentials. See this blog post for full details.