On a default Thunderbird installation your mail goes into ~/.thunderbird - by making a backup of that directory you can restore it anytime.
If your /home directory is on a different partition anyway, then you only need to make sure that the /home partition is NOT formatted when installing FC10. When doing an upgrade, rather then a clean install, then your /home directory should remain instant. But in case of an upgrade I'd rather advise to do an FC9 upgrade first, before upgrading to FC10. This is a safer route for cases when applications and configuration concept change over time - subsequent FC version are usually more compatible.