One of few downsides of Ultrasurf is that it does not automatically configure Firefox. (This is the case for most of the non-VPN based hide IP tools. But there are always exceptions, like IP Privacy.)
If you want to browse using Firefox and being anonymized by Ultrasurf you have 2 options:
Install the Ultrasurf Firefox add-on (see how)
Manually [...]
How to configure Firefox to work with Ultrasurf