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Step 1. Find a list of free proxys. You can Google it or use this list here.
Step 2. Now choose an IP and PORT from the list. In this tutorial I have chosen IP: 200.3.170.189 and port: 8080. You can choose whatever you like.
Step 3. Open Firefox and in the menu choose Tools and then Options
This will open the Firefox options window. Click the Advance options.
Step 4. Click on Network tab and the click Settings button under Connection.
Step 5. In Connection Settings window, click Manual proxy configuration. Now enter the IP address in HTTP Proxy and Port address in Port. ( In this example HTTP Proxy address is 200.3.170.189 and Port is 8080).
Also check the box 'Use this proxy server for all protocols'.
now just hit ok :D and your IP is set to desired Proxy.