I do not understand what you are saying, geffr. Are you saying that you gave instructions, somehow, and that you were overridden? As far as I remember, if you let a program do what it is set up for, it will do everything, you like it or not. I can't remember a single case where it was possible to tell: Import!, and that the program will give you options. So, if you had 10 GB of mails in 4.79, you will get 10 GB in TB.
Of course, a saner way would be to set up an account in TB, and download what is on the server ( in my case only the last ten days' of mail. If you like what you see, later, you can copy/paste what you need.
It, surely, is not necessary to stick to an ancient version of Netscape!
UserAgent: Mozilla/5.0 (Windows NT 5.1; rv:13.0) Gecko/20100101 Firefox/13.0.1