Collect the correct details
You normally need a hostname or IP address, a port, and sometimes a username and password. Confirm whether the proxy is HTTP, HTTPS, or SOCKS.
Enter the settings
On Windows, open Network & internet settings and locate Proxy. On macOS, open Network settings, select the connection, then view proxy details. Firefox can use its own proxy configuration under Network Settings.
Test carefully
Open a low-risk website first. Check for certificate warnings, unexpected redirects, missing images, or login prompts that look different. Remove the proxy immediately if behaviour is suspicious.
Remove or troubleshoot
Return to the same settings and disable manual proxy use. Verify the host and port, check authentication, and confirm that a firewall or workplace policy is not blocking the connection.