Once on the archive site, look for the /pub/thunderbird/ directory.
Inside the version folder, choose your operating system: win32/ (Windows 32-bit) win64/ (Windows 64-bit) linux-x86_64/ (Linux 64-bit) mac/ (macOS)
If a new update breaks a critical add-on or isn't compatible with an older operating system (like Windows 7 or older macOS versions), you can find the exact previous build that worked for you.
Some modern browsers have disabled the ftp:// protocol. If a link doesn't work, ensure you are using the https://mozilla.org prefix instead.