Firefox, as I mentioned, is the default browser that comes with Ubuntu, but it is by no means your only choice. If you like Firefox, you might also want to look at Mozilla (http://www.mozilla.org) as a full-featured browser suite featuring an email client, HTML editor, and more. Konqueror, another excellent browser, comes as part of the KDE desktop but you don't need to install KDE to take advantage of it. Just search for Konqueror with your Synaptic package installer.
We can't stop there. You also have access to some text-only browsers such as Lynx and Links. Neither of these comes pre-installed with Ubuntu, but both are easily added if you are curious.