All versions are available form here: https://archive.mozilla.org/pub/thunderbird/releases/.

As you haven't said which operating system you are on or what language you prefer I can't narrow it down further for you.

Windows11 and English. Thank you.

Before you even consider this be aware your existing profile of contacts and settings will not go backwards with you. See https://support.mozilla.org/en-US/kb/dedicated-profile-thunderbird-installation

Perhaps explain why you think this is a good idea to go back. It may be you can avoid loosing your information by staying on 128.

American English: https://archive.mozilla.org/pub/thunderbird/releases/115.14.0/win64/en-US/Thunderbird%20Setup%20115.14.0.exe

British English: https://archive.mozilla.org/pub/thunderbird/releases/115.14.0/win64/en-GB/Thunderbird%20Setup%20115.14.0.exe

Canadian English: https://archive.mozilla.org/pub/thunderbird/releases/115.14.0/win64/en-CA/Thunderbird%20Setup%20115.14.0.exe