Allowing Burp to listen to localhost in Firefox

When security testing a web application it is often useful to proxy the HTTP traffic from the web browser through a tool like Burp Suite. Burp lets you inspect the traffic from the browser, modify it, resend it and do various other interesting things with it.

Configuring Firefox to use Burp as a proxy is straight forward and described well here. However, when testing an application running locally on your own machine, there seem to be a specific setting that needs to be enabled for this setup to work. To enable this go to about:config in Firefox and make sure the setting

network.proxy.allow_hijacking_localhost

is set to true.

After that, the traffic should be proxied to Burp as expected!