I'm getting linux and don't want to lose my e-mails in the format.

I can type in a long tutorial on how to move/backup thunderbird profiles but perhaps it's better to have a look the following page as it explains all about Thunderbird profiles.

http://www.mozilla.org/support/thunderbi...

Basically you need to....
1) Locate your current profile directory
2) Back up your profile (in your case to external HD)
3) Format internal disk and install Linux
4) Install Thunderbird under Linux
5) Create a new profile using profile manager (thunderbird -P)
6) Point folder for your new profile to your backup directory