Which Machine Does localhost Mean Through a Proxy?
When a proxy opens the destination connection, a loopback destination refers to the proxy's environment. Browser bypass rules can hide that distinction.
You want to visit a development server on your laptop while using a remote proxy. The word localhost needs an owner.
On a direct request, the client's loopback destination is local to the client. If a proxy actually opens the destination connection to a loopback address, that address belongs to the proxy's environment instead.
A local proxy and a local web server can both run on the same machine, but they still need distinct listening endpoints. Entering the web server's port as the proxy port does not make it speak the proxy protocol.
Browsers also apply bypass rules, including implicit exceptions for some local destinations. Firefox's loopback protection is separate from its ordinary bypass list. A page loading at localhost therefore does not establish that the remote proxy carried it.
Decide the intended route. A client-local development site commonly needs an appropriate direct exception. A service that must be reached from another machine needs an address and access arrangement valid from that machine. Changing the displayed proxy IP cannot make the remote machine's loopback become your laptop's loopback.