It's funny how many people are always saying, is there a Trillian for the Mac or something similar, when Fire was out on the Mac before Trillian was released. In fact back then I think the only other multi-protocol messenger was a program called Everybuddy for Linux.
Anyway, I started using Fire with the public beta, and continued to use it through 10.3, but then I got sick of how much it was crashing, so I tried out Adium. I didn't really like Adiums messaging window, I liked the simpleness of Fire's, and I tried lots of skins and never found one I liked. So now with Tiger I'm using iChat and a Jabber account to connect to MSN and Yahoo.