Readme: How to disable Firefox's automatic captive portal detection.

README.md

@@ -477,6 +477,11 @@ Otherwise you may manually [modify your browser's settings][Firefox proxy] to:
 [Firefox proxy]: http://www.wikihow.com/Enter-Proxy-Settings-in-Firefox
     "How to Enter Proxy Settings in Firefox"
 
+Additionally, you may want to disable Firefox's automatic captive portal
+detection so that you get less noise in client logs.  Enter `about:config` in
+the location bar and change `network.captive-portal-service.enabled` to
+`false`.
+
 Once done, you can visit `localhost` in your browser and it should show you
 the *client front-end* with assorted information from the client and
 configuration tools: