Mibbit is still proprietary which is the same issue with Slack. The KiwiIRC webbased IRC client is open source with a free to use (no ads!) hosted version that

Mibbit is still proprietary which is the same issue with Slack. The KiwiIRC webbased IRC client is open source with a free to use (no ads!) hosted version that is gaining in popularity. More and more existing IRC networks are enabling Kiwi with complete Freenode support also. UI improvements are coming up to make the most IRC actions usable via the UI which will make a huge improvement over many IRC options out there. Disclaimer: I'm a Kiwi IRC dev.