All versions of Thunderbird can be found here. http://ftp.mozilla.org/pub/thunderbird/releases/

To make it easier for you see https://ftp.mozilla.org/pub/thunderbird/releases/78.14.0/mac/ or https://archive.mozilla.org/pub/thunderbird/releases/78.14.0/mac/ and pick your language. If you use a bit more current macOS that is 10.12, 10.13, 10.14 then Thunderbird 115 and Firefox 115 ESR will be the most current version you can run as 116.0 will require macOS 10.16 at minimum to run. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

Matt and James thank you !