am not going to give detailed or necessarily lucid instructions/steps, am writing this more for myself, just as a reminder, as i’ve had to do this two times now in the past month.

1) you’ve got outlook installed, been using it for mail, on your pc
2) install thunderbird on the pc
3) once thunderbird is installed, import (in the file menu) mail from outlook. if you’ve got a lot of mail, it takes a while
4) check the mail to make sure it’s all there
5) find something that looks like this url on your pc’s hard drive:

mailbox:/C/Documents and Settings/yourusername/Application Data/Thunderbird/Profiles/q3feqayh.default/Mail/Local Folders/Outlook Mail

6) you may need to make hidden files visible to find this
7) zip up the Profiles folder
8) use whatever transfer device you want, hard drive, media card, just transfer that zipped Profiles folder over to your mac (i use the 2gb compact flash card i use in my camera)
9) i’m assuming you already have thunderbird installed on your mac
10) open up the zipped Profiles folder on your mac, and understand at that point that you didn’t necessarily need to’ve zipped the entire Profiles folder, just the profile within it, but you did it so you’d be able to objectively compare file structures between the two Thunderbird installs, because you’re still trying to figure out how to finish this
11) very very confused for about twenty minutes, trying out various methods, until you realize you need to take all the files within the “Local Folder” file from the unzipped file that have “Outlook” in the name, and move those files as a group to the new “Local Folder” on your Mac, which is roughly at username > Library > Thunderbird
12) you have to close Thunderbird and then reopen to now have that mail show up in Thunderbird, and the mail from your PC will now be subfolders under “Local Folders” in your Thunderbird “All Folders” menu.

and if you’ve done this several times, back and forth, like i’ve had to do 3 times this year, and haven’t cleaned up after yourself each time, you’ll find all that mail in the form of subfolders under “Local Folders” in Thunderbird is a huge mess, and it’ll take you a while to clean it up.

again, these notes are for myself, so no lucidity or follow-ability is guaranteed.