I noticed this problem last night; when firefox 2 is not already running, trying to use /url -n in mIRC will freeze. I'm not sure if this is an issue with mIRC (I'm using 6.2) or FF2 though. Or just a random thing that started up.

I knew I shouldn't've have rebooted the other night

Check to make sure that FF isn't still running. Apparently, FF doesn't always close properly and then when you try using a url from mIRC, it causes mIRC to freeze. Check your task manager processes to make sure it's not still running. Make sure that you click the processes tab and not just look at the applications tab.

It did come up with a complaint that firefox was still running, "hung" but I thought it had done it again after I killed the process in taskman. I'll check again.

That does seem to be the problem. Thank you Now I just have to figure out how to batter firefox into closing itself properly.

File -> Exit
or the equivalent shortcut. This is a good habit to get into for instance if you set Firefox to restore all sessions and windows on restart.

If you continue seeing a firefox.exe process it's possible you may have Poison Ivy trojan that is evading detection - http://forums.mozillazine.org/viewtopic.php?t=467349