Why would you want to uninstall Thunderbird in the first place?

uhh because I don't want it anymore, christ...

and your going to read the emails that are left how? Not many mbox readers out there.

Or are you looking to just make sure they are gone? in which case type %appdata% into the search box and delete the Thunderbird folder from windows explorer.

Best of Luck with Windows 8. Maybe Window 9 will be useable.

If whatever new email client you go to does not import your TB messages, you can leave TB on your system and just use it to read the emails it has.

If you do this, it would help to set TB to not check for new messages: Go to Account Settings > Server Settings. Under each account untick all the boxes so TB never checks for messages. (Clicking the 'Get Messages' button will still get messages.)

You can also set TB Offline: Go to File > Offline > Work Offline.