well, this may not be exactly what you're looking for, but there is a version called Knoppix that boots from a CD, if you want something that's not permanent and will help you get the feel for it a little bit. By the way, I know very little about linux, but knoppix is pretty cool. As for firefox, there is a version you can download for linux. Also, as for Instant messenging, there is a program similar to Trillian called gAIM. Good luck
Forgot to add: Knoppix is based on Debian, which is one of the major distros, some other major ones are Red Hat, Mandrake and SuSe.
You can search for the right one
here