They should be saved, yes. Thunderbird stores messages, contacts etc (the user data) in a folder called a 'profile'. It's in a different location to the program. When you reinstall the program, it looks in the default location for a profile folder, and if it finds one already there it uses it. (if it doesn't, it creates a new blank profile.) For peace of mind you can backup the profile on external storage like a memory stick (that's good practice anyway). Then you can install the latest Thunderbird and, if necessary, copy the profile back on to your computer. Instructions are here: https://support.mozilla.org/en-US/kb/profiles-tb