I avoid browser/mail clients almost on principle (it sounds as if you're actually using the Mozilla mail portion, which I've had lock up on me when I've just tried to send Email.)
What I've done in installing various versions of Mozilla is install it into an entirely separate directory (eg, M.98, M.99, M1.0), keeping the old directories around until I'm reasonably sure the newer one is working, then I make sure any old files it might be using are transfered to the new Mozilla directory, then I make a "trial balloon" of deleting the older version by putting it under, say, a "savedmoz" directory tree. It's been working reasonably well. (Actually, I do all this stuff because I almost *never* trust a program under Windows to be able to upgrade itself in some reasonable sense of order, so I get all paranoid.)
Much as I hate to admit it, I am using Microsoft Outsnort Express as my "main" Email program at home, and they've standardized on Outsnort at work (though I know a guy at work running Novel Unixware who has circumvented that by using some client or another.)
And I use neither pile of dog vomit news group readers, of course. I'm currently using a program called "Xnews" which, despite its name, has nothing to do with XWindows or X-anything.