I use Firefox exclusively, except when I run into one of those pages that will only render properly in IE.
For IM, I use Trillian. It's a tool that all of the tech dudes use (my SO is a computer guy, so he sets me up with all of the newest tools and things) You can use whatever account you currently have for IM (MSN, AOL, or whatever), and have only one window where you can talk to all of your IM contacts. You can also set up security, anti-virus and encryption.
I don't know a lot of the technical details, but it's a free add-on that you can download.
Trinity