HydraIRC is abs. free and as good as Mirc when not better, or do yourself a favor and install Opera, not only the best free browser in town (speed, security, and stability), but it also futures a built-in IRC client .
Firefox has become a memory hog piece of garbage, it's hard to say but even IExplorer 7 beats the crap out of it.
How do I know? I tested them all.