Open Firefox browser and open preferences window by selecting the menu as shown below. Press “Edit” in the menu bar and then press “Preferences” item in the menu.

This opens up Firefox’s preferences window. In that select “Network” tab under “Advanced” section as shown below.

Then press the “Settings” button to open “Connection Settings” window as shown in the below diagram.

If you have proxy server IP address and port number, just select “Manual proxy configuration” option and enter them in HTTP proxy and port text fields respectively. If you use exactly same proxy for all connections like HTTPS, FTP etc then select More >