Another option, and some suggestions.
Rather than changing browsers, it might be better to change e-mail clients.
[link|http://www.pmmail2000.com/|PMMail2000] can import Netscape 4.x e-mail folders, etc. It's shareware, like Opera. Eudora's very nice too, and some like [link|http://www.pmail.com/|Pegasus Mail]. Pegasus is free.
Personally, I think something else is up with your system that your problems with Mozilla are just a symptom of. But, perhaps your upgrade of Mozilla did get mangled a bit. Mozilla's readme.txt does say (for an RC1 nightly I have):
For all platforms, install into a clean (new) directory. Installing on top of previously released builds may cause
problems.
I imagine you've only installed one version of Mozilla, but if not that might be something to consider.
You might also look around in ..\\Mozilla\\RC1\\defaults\\pref\\mailnews.js (use the appropriate directory tree for your system). It's the default script that controls the Mail function of Mozilla. (It's mostly greek to me.)
I notice in [link|http://z.iwethey.org/forums/render/content/show?contentid=34423|this] thread that you were having problems with Mozilla displaying pages that others weren't having. Mozilla (RC1 on a WinMe box) using [link|http://sourceforge.net/projects/ijbswa/|Privoxy] (nee JunkBuster v2) as my proxy works fine here. I'm not using 98lite nor anything similar (though I do have lots of the extra eyecandy in Me turned off - e.g. I use the "classic" desktop and the "classic" folder types). I haven't turned off much in Mozilla - I let the proxy take care of blocking things - e.g. the only thing I mess with in Mozilla's Advanced settings is the proxy page.
Summary:
1) Run "scandisk" to make sure nothing's mangled on your disk.
2) Reinstall Mozilla in a fresh directory. Then import your bookmarks, mail, etc. Try it with and without your proxy software.
3) Only then, if you still have problems you can't resolve, investigate Opera and/or a new mail client like Eudora or PMMail.
My $0.02. Hope this helps a bit. Good luck!
Cheers,
Scott.