It follows the same version numbering cycle of Firefox ESR (Extended Support Release), just without the intermediate versions.

Because codebase is synchronized with Firefox, but Thunderbird team doesn't have the manpower to release a new version every 6 weeks like Firefox, so they only release the versions which correspond to Firefox's extended support releases, which happens to be every 7 Firefox releases.

24, 31, 38 represent the version of Gecko that Thunderbird ships with.

https://wiki.mozilla.org/Thunderbird/New_Release_and_Governa...

Thunderbird now follows an Extended Support Release process, rolling out every eight release cycles equaling every 54 weeks.

https://www.mozilla.org/en-US/thunderbird/organizations/faq/

This is because Thunderbird is no longer in active development; it is primarily in maintenance mode.

https://blog.lizardwrangler.com/2012/07/06/thunderbird-stabi...

To keep parity with the version of Gecko engine that it uses I believe.