apenwarr: I know what you mean about the old monolithic mozilla, but phoenix^Wfirebird has a couple of nice UI innovations that mean there are genuinely useful additions for everyday users. The main one, I think, is that the popup disabling stuff is pretty much automatic. Your grandma may not know what a popup is, but I'm sure she's irritated by whatever the hell those funny windows are. I imagine that thunderbird's anti-spam training, with a better UI and/or explanation would be a killer too.
Spam and popup disabling are the two biggest peeves I've seen people have with the Net these days. The mozilla project goes as far as anything I've seen to fixing those problems for everyday users. The UI could do with some tweaks, but it's getting there. And people are seriously motivated to fine solutions to these problems - it's amazing watching one person in the office discover a solution, and teach it to everyone else they know for instance.