THIS IS FOR EDUCATIONAL PURPOSES!
1. Find a list of free proxy. You can Google it.
2. Choose IP Address
Now choose an IP address and port from this list.
Example IP: 196.333.22.1 and port: 8080.
3. Firefox > Tools> Options
Open Firefox and in the menu choose Tools and then Options. This will open the Firefox Options window. Click on Advance Options.
4. Network Tab > Connection
Click on Network tab and the click Settings button under Connection.
5. In Connection Settings window, click Manual proxy configuration.
Now enter the IP address in HTTP Proxy and Port address in Port.
Also check the box 'Use this proxy server for all protocols.
Click ok and your IP is set to desired Proxy.
6. Done

Now you can go a website you want with a different IP. :evil: