A simmilar thread was posted by Jodosha a few hours ago. More information on the bug can be found in that thread.

Aside from the bugfix, v6.0 has a couple of neat new features - including this one:

Taken from mIRC news:
(...) Multi-Server support has been added. With this feature mIRC allows you to connect to multiple servers at the same time. (...)
No more opening 3 instances of mIRC to connect to your favourite servers - yay!