This address uses a network port which is normally used for purposes other than Web browsing. Firefox has canceled the request for your protection.

This message appears if you want to access a website on a non-standard port, e.g. http://example.com:42 .

Being a developer you might run in your non-productive environment a web server listening on such a port. As a workaround you can tweak your firefox by adding the configuration network.security.ports.banned.override to your preferences.

To do so go to about:config, klick on the “I’ll be careful, I promise!” button and right-click any preference. In the appearing context menu click New > String and add the new preference

network.security.ports.banned.override

with a value “1-65000″ or whatever ports you want to access. They don’t describe blocked ports but ports that are considered to be not banned.

I recommend to create a new firefox profile for development and testing purpose and that you only open ports that you really need.