The old Tb 115 esr has not been supported with updates since 115.18.0esr in December 11, 2024 as to why it may not be an option on https://www.thunderbird.net/thunderbird/all/ You can however download Thunderbird 115.18.0.dmg for macOS from https://archive.mozilla.org/pub/thunderbird/releases/115.18.0/mac/en-US/ or if you prefer another language https://archive.mozilla.org/pub/thunderbird/releases/115.18.0/mac/ The older Firefox 115 ESR is still supported with updates (till Fx 115.27.0esr in August unless updates are extended again) so it is still possible to get it at https://www.mozilla.org/firefox/all/desktop-esr/osx/ as pick your language and select Download ESR 115. I see you may have this already.