It's now easy to use your proxy on Firefox with the FoxyProxy addon. It is easy to install and setup
1. Open Firefox and enter about:addons in the address bar, it will open the Addon manager
2. Under the search bar, enter FoxyProxy
3. Click on FoxyProxy Standard and then click on "Add to Firefox
4. After adding the addon, click on it from the tool bar as shown in the below image and then click on Options
5. This will open FoxyProxy manager, click on Edit
6. Select the Proxy Type (HTTP or SOCKS5) and then enter the following details:
Title or Description : Anything of your choice
IP address, DNS name, server name: The proxy IP address
Port: 1212 or the SOCKS5 port number
Username: In case you are using username authentication, add the user ID, else leave it blank
Password: In case you are using username authentication, add the password, else leave it blank.
7. Click on Save and click on "Turn off FoxyProxy...", then select the proxy that was just configured from the drop-down menu
You can now access https://www.ipchicken.com/ to check if the IP has changed.