Unlike Chrome, Firefox maintains its own independent proxy settings rather than relying entirely on the operating system, which makes it easy to run a different proxy just for Firefox.
Where to find it
Open Firefox Settings, scroll to Network Settings near the bottom of the General tab, and click Settings to open the connection configuration dialog.
Manual configuration
Select 'Manual proxy configuration' and enter your proxy's address and port in the appropriate field (HTTP, SSL, or SOCKS depending on your proxy type). Firefox lets you set different proxies for different protocols if needed, or check 'Use this proxy server for all protocols' to keep it simple.
A useful exception field
The 'No Proxy for' field lets you list domains or IPs that should bypass the proxy entirely, which is handy for keeping local network access or specific trusted sites working normally while everything else routes through the proxy.