I have tons of mail saved in Thunderbird but would like to consider using emacs for reading/writing email. Can any one of the existing emacs email packages (e.g., rmail, gnus, VM, wanderlust, ...) use this archive and also save/store future correspondence in this mail format?
Here is a thread on Thunderbird-to-emacs migration. It notes a couple relevant points:
Personally, I have tried several emacs mail clients, and mew (which also supports mbox) is what I've settled on. Whatever client you try, expect to spend more time configuring it than you would, say, Thunderbird. Things like "save my sent messages in this folder" and "properly display HTML email" probably won't work like you want them to "out of the box" like a conventional mail client. IMHO, it's a worthwhile investment.