Import mail boxes from Thunderbird crashes mail!

Has anyone any idea what I can do to ensure Mail does not crash when I import mail boxes from Thunderbird?
I would love to us Mail but unless I can bring my mailboxes it isn't happening.
Is there a file size limit or is Mail for Mac just flakey?


Could you please explain the whole process of exporting mail from Snow to Lion. I have been trying for weeks on two diffrent computers and I can not get it to work.
I have moved the library, not sure to delete the envelopes. How do you export the  mbox-files ? Any help would be greatly appreciated.
Thanks in advance!