It said Firefox doesn't know how to open something like that. And I do have Firefox. I tried it with internet Explorer, but that does not work now. What shall I do?
Delete this. I figured it out myself.
When you go to the site irc://irc.mpgh.net It will ask for you to open it using a program. Which you don't have. So I suggest using a Runescape client such as Swiftkit if you play it. Or get smarter by downloading mIRC from mIRC: Internet Relay Chat client