This document is on how to get Firefox and other Mozilla browsers to go to http://mibbit.com for irc:// links. This document also shows what type of links work. If looking for a spec for Irc:// links, there is no official one, but there exists a spec-like suggestion: Irc:_urls_in_Mozilla.
Getting Firefox to recognize Mibbit as IRC handler
Head to Mozilla Links for an article by Percy Cabello on how to do this with pictures that explain it clearly.
Other than that, if you are logged in, go into your preferences, and below the buttons for Save and Close, Reset All To Defaults, & Load a Skin, there is a link to allow Mibbit to be the protocol handler.
Currently, Mibbit only has partial support for IRC:// links. What currently works is as follows:
irc:// -- Takes a person to http://mibbit.com without a channel or server. If you are logged in, will take you to your normal channels and servers.
irc://irc.servername.com:6667/channel -- Has http://mibbit.com open and open the server 'irc.servername.com' under the port 6667. The channel 'channel' is open, and is optional. Any port that the server allows can be used, including secure ports.
ircs:// -- Any Secure irc link fails.
irc://irc.servername.com/channel& -- Mibbit does not yet allow parameters.