Firefox's Loopback Protection Is Separate from Its Bypass List

Firefox has a loopback protection check before its ordinary proxy host filters. Removing localhost and 127.0.0.1 from No proxy for does not disable that separate check.

For an intentional local-development capture, configure the proxy and the relevant bypass entries, then open about:config and set:

network.proxy.allow_hijacking_localhost = true

The preference allows loopback hosts to proceed to the normal proxy-selection policy. An explicit matching bypass or a PAC result of DIRECT can still choose a direct route.

This changes whether the browser may send that destination through the intermediary. It does not make the local web application reachable from a proxy on another machine: that proxy's localhost identifies the proxy machine.

Use the proxy and destination arrangement intended for the development task, then restore the preference when that exception is no longer needed. The setting does not select a proxy endpoint or repair an unavailable listener.