Since those emails were not backed up and were lost when you upgraded to W7 my verdict is they are gone.
Moving said emails to the Apple might be as simple as copying the Thunderbird files to the folder that the Apple Thunderbird version uses (is there such an app?) If not you have to install Windows on the Apple, install Tbird and then move those files over.
I have a dead HP laptop, but the hard drive is fine. It came OE with Vista, but I upgraded it 6 months ago to W7. I just bought a MacBook Pro, and a hard drive enclosure. I downloaded Thunderbird to an older XP PC and found the Vista e-mails on my HP HD, but I can't find any e-mails for the last 6 months( since I upgraded to W7). I really want to find my most recent e-mails and move them to my Mac! Can anyone help me?