Get the release version and install it. https://www.mozilla.org/thunderbird/all/

Many thanks - Jon

This seems to be a bug for what the start page displays if you have that option selected. There are numerous identical questions on this today. I enabled the Display Start page and restarted thunderbird and got the same message. I downloaded nothing. I am still on the same version 45.1 that was on before enabling the start page to display. Either turn off the start page or ignore it.