My Ubuntu Thunderbird is likewise at 68. The repositories don't seem to get updates in a timely fashion, but in some cases like this it's a blessing in disguise. :-)
For example, for Ubuntu 64-bit: https://ftp.mozilla.org/pub/thunderbird/releases/78.4.0/linux-x86_64/en-US/
Of course, it's just the application as a compressed tarball. The extra stuff to hook it into the OS and your favorite desktop environment is manual since that's individualized.