The latest Firefox 3.5 nightlies now include Mibbit as a bundled IRC protocol web client so now, when clicking on an irc:// (or its encrypted variation, ircs://) link, it will be redirected to Mibbit so you can connect to the desired server using a web interface.
Protocol web handlers support was introduced in Firefox 3 and allows Firefox to redirect a protocol request to a standards compliant web service that is capable of parsing and serving the request. Firefox 3 included support for Gmail, Yahoo! Mail and 30boxes as email and calendar providers.
Like with other bundled providers, you will have to explicitly select it from the Applications page in the Options dialog.
After using Mibbit’s great service for years, I am glad to have filed the bug about it and to see it come closer to my browsing.
If you want to add Mibbit right now, follow the instructions in this previous article.