Below you'll learn how to change IP address in Firefox by using a special type of Internet server known as a proxy. Access to these server is available on both free and paid systems. I'll discuss the differences in this article as well.
STEP #1 to change IP address in Firefox
Go to your Firefox's dropdown menu for Tools -> Options
STEP #2 to change your IP address in Firefox
Click on the Advanced tab, and then on the Network tab.
In the "Connection" area, click Settings.
STEP #3
Check the box for "Manual proxy configuration".
Check the box "Use this proxy server for all protocols"
In the box "No Proxy for", enter "localhost, 127.0.0.1" so that connections within your computer don't go through the external relay.
See more details with diagrams at http://tecno-updates..com/2013/11/how-to-change-your-ip-address-with.html

I hope this post is helpful

Thanks for your useful information.You can also try these ways given below
Follow the below steps for changing IP address in Windows XP:
1.Start->Run->Cmd->Press Enter
2.Type in "ipconfig/release" for Current IP address and
3.Then type "ipconfig/renew" for New IP address .
You can also change your IP address by this way,
If it is static IP,you cannot change it.Static IP will not change even when you reset the modem.You have to contact your Internet Service provider(ISP) for changing static IP.If your IP address is dynamic,it will change whenever you reset the modem.You can check your IP address by using Ip-details.com .