At https://www.thunderbird.net/en-US/thunderbird/78.14.0/releasenotes/ the download link is for 91.1.0, not 78.14.0
I'm on an old MacBook Pro running MacOS 10.9.5. I am running TB 78.13.0. TB is telling me to upgrade to 78.14.0 but the Download button is for 91.1.0 (and that requires a version of MacOS that won't run on my MBP).
I know I can manually edit the https://download.mozilla.org/?product=thunderbird-91.1.0-SSL&os=osx&lang=en-US link so it says https://download.mozilla.org/?product=thunderbird-78.14.0-SSL&os=osx&lang=en-US, but the Download button on a release notes page should really just download the version described by those release notes.