To configure HTTP or Socks proxy in Mozilla Firefox browser on macOS, open the Settings section in the upper right corner.
Scroll down and press the button.
Configure proxy in Firefox browser on macOS.
If an HTTP proxy with authorization is used, then a window with a login and password will appear as soon as you open any site on the Internet.
Browsers do not support Socks proxies with authorization. In such cases, it is necessary to use proxy programs. For example, Proxifier.
More about setting up Proxifier on macOS.