Simple solution. Go to Tools, click on Options, the last option of pop up menu. There, click on General, then on Connection Settings. There you have to type the required IP address and Port number.
I attached the images so you can understand better. Any querry still tumbling in your mind, you may ask.
Click Start
Click run
Type cmd (if you are using winxp) and press enter, OR if you are using win98 type winipcfg
a black box will appear. type ipconfig/all
you will see the Default gateway of your dialup or lan what ever medium you are using . Put that Ip in your firefox connection tab.