By default Firefox does not use a proxy host or a proxy port. A proxy is only used when internet traffic needs to be routed through a certain computer or network device. A proxy is commonly needed in business environments but not in home environments to get online unless someone setup a proxy server for you. If your in a business environment I would strongly suggest you contact your IT department as proxy server settings vary from network to network. If your on a home network are you trying to get Firefox to use a proxy?

yes, i trying to get firefox to use a proxy (as ultrasurf)

Once you have Ultrasurf setup you can go into the Firefox proxy settings and set the Proxy Host as 127.0.0.1 and the proxy port to 9666. Please see this web page for a step by step guide http://www.how-to-hide-ip.info/2009/02/19/how-to-configure-firefox-to-work-with-ultrasurf/ .