The
Proxy realm field is internally pre-populated with the value
moz-proxy://[HTTP Proxy]:[Port]
. If the field is empty, this value will be used when authenticating the browser. If the proxy expects another unknown value for the proxy realm, you can determine this as follows: Leave the
User name and
Password fields empty and launch the browser. The dialog window which appears will contain the correct value for the
Proxy realm field:

In the example above, the value for the
Proxy realm field is as follows:
moz-proxy://172.30.178.10:8080