Thunderbird ships alongside Firefox ESR releases, based on the ESR Gecko branches. The version matches the Gecko version. Gecko 60 was an ESR, as is Gecko 68.

Thunderbird ships alongside Firefox ESR releases, based on the ESR Gecko branches. The version matches the Gecko version. Gecko 60 was an ESR, as is Gecko 68. So Thunderbird 68 is the next major release after Thunderbird 60. Until today, Thunderbird 60.0.something was the latest release, and Ubuntu is not so out of date.