How to configure Burp Suite for localhost application
If none of these solutions work for you, like they didn't work for me, you could try to change value network.proxy.allow_hijacking_localhost to true (using firefox v 67.0.1 64-bit).
Open new tab, type about:config in address bar, then type network.proxy.allow_hijacking_localhost and double click it to change its default value to true.
In Burp go to Proxy / Options / Proxy listeners, and confirm the Running box is ticked.
In proxy tab make sure intercept is turned off.
Make sure the proxy in burp listener is 127.0.0.1:6666.
Configure your browser to use 127.0.0.1:6666 as its proxy.
Configure Burp to use your original LAN proxy (from your original browser configs) as its upstream proxy.
Run your browser and access your application.
Update: Additional reasons could be browser restricted ports. You mentioned you are using Firefox. Try this.
- Type the following into Firefox address bar 'about:config'
- Type the following settings 'network.security.ports.banned.override'
- Enter your port number '6666'
Try adding a '.' after "localhost". This should solve your problem.
eg.
http://localhost.:8080/WebGoat/ This will force the localhost to use the same proxy settings as one would with an internet connection/adapter