I use both Mail.app and thunderbird. You should be able to copy your thunderbird backups to the Mac. It goes in ~/Library somewhere.
Fire.app is the best multi-protocol IM client out there, but Adium is nice too.
Mac OS X has Samba built in, so you can share folders to Windows machines and access their shares.
You can minimize a single window with Command m. Command being the apple key. command tab I think lets you move between applications, and Command ` lets you switch between Windows in a single Application. I think.
EDIT: Oh, and Camino kicks the crap out of Firefox.