older versions are always available at https://ftp.mozilla.org/pub/thunderbird/releases/ just pick the release, the OS, the language and you're good to go. However, you may be better served if you share some of the details. If it's threading you do not like, that can be easily remedied. But, I respect some users prefer older versions. They still work, but, with a little customizing on your part, I think you will prefer the newer releases.

David did not say how to remove threading, but following his encouragement that it could be done, I think I found the answer to turning off the threading in Thunderbird 128. 1. Went to "Settings" 2. In the section "Open messages in" I changed the selection from "a new tab" to "an existing message window.

So far after the above change, my Thunderbird 128 acts like my old Thunderbird 60 in that threading is not displayed. And now when I click on the X to close an email it just closes the email and not all of Thunderbird like it used to.