It appears freenode has banned the Mibbit client. I guess now you have to use theirs
I think freenode is totally shady area for doing this.
They say they baned mibbit because many trolls use it (and mibbit folks did not help them)
But i feel the real reason is on personal level with axod (mibbit main dude) and the fact that mibbit for profit company with closed sourced IRC client. To some degree mibbit became just a scapegoat for people to blame resent troll activities on too.
I had to switch from using mibbit to other irc client because of this.