Tbird:
highlight a folder
Tools - Import/Export in mbox/eml format - Export all messages in the
folder - EML format
pick a location to save the files
OK
the files will be saved in a subfolder
(do this for all the folders)
WM/WLM:
File - Import - Messages - Windows Mail 7/Windows Live Mail
select the folder location you saved the files to
next
All folders
next
Finish