Use a Proxy IP in Firefox
Match an HTTP or SOCKS entry to Firefox's connection settings, check the browser's exit address, and undo the setting afterward.
Firefox has its own connection settings, so you can test a proxy without changing the system setting used by other browsers. Have the IP address, port, and protocol ready before opening the dialog; a free proxy list provides entries to evaluate.
Find the connection settings
Open Firefox Settings and search for proxy or Network Settings. Open the connection settings and choose Manual proxy configuration. The exact wording can vary slightly by release and language.
If the controls are locked or an extension is shown as managing the setting, check that source of configuration first. Re-entering the IP will not necessarily override it.
For an HTTP proxy
Put the address into HTTP Proxy and the port into its adjacent port field. For the documentation example 203.0.113.10:8080, those values are 203.0.113.10 and 8080. Substitute your actual endpoint.
If Firefox shows separate HTTPS or SSL proxy fields, enter the same IP and port there, or select the option to use the HTTP proxy for HTTPS too. The endpoint must support CONNECT for HTTPS destinations. These fields choose which traffic uses the endpoint; they do not mean that its proxy URL must start with https.
For a SOCKS proxy
Use SOCKS Host rather than the HTTP Proxy field. Enter the endpoint's address and port, then select the matching SOCKS version. A SOCKS4 entry and a SOCKS5 entry are not interchangeable just because both use a common port.
Review the option for proxy-side DNS if you want Firefox to send destination hostnames to the SOCKS proxy. Check the actual setting instead of assuming a default from an old guide or from another browser.
Check the result inside Firefox
Save the dialog and open an IP-check page in the same Firefox profile. Compare the reported address with a direct request. If the page does not load, test a second entry of the same protocol or use a short command-line request to separate an endpoint failure from a browser setting problem.
Look at the No proxy for or bypass list when one destination stays direct. Browser extensions can also control proxy behavior. These are different explanations from a dead proxy IP.
Restore the previous mode
When finished, return to connection settings and restore the original mode. That may be No proxy or Use system proxy settings; choose the one the browser used before your test. Close or reload pages so subsequent requests use the intended route.