Firefox, actually. (Which is the new program-with-a-million-extensions.) I just use gmail for mail these days, though I would probably go back to mh or mutt if I started using a client program again.
I've never found an RSS reader I've been satisfied with, though. I was working on one at one point that had an interface like mh, but I haven't gotten back to it in a while.
I have never encountered a system that could replace Gnus, though.
What do you use to read nntp, email and rss in a coherent interface?