What are you trying to do?
If you use Preview button that would automatically open a browser. Worst case just run NodeWebkit as the browser to test. No need to touch the firewall.
If however you are trying to preview or LAN. Then you should use the local IP of the computer that has C2 and previewed(at least the current build once). For example my dev computer has the ip 192.168.0.100 so my mobile devices and other computers need to put in "192.168.9.100:50000"
Also it is important to turn on preview over lan in the options menu.
If LAN is not your problem. Then press the Preview Button.
There is some tutorials to help with the different preview method in the tutorials section.
Also this section is for bugs, not a section for having troubles.