Since Mozilla Thunderbird 115 is now ESR, what's the difference between 115 ESR and 128 ESR?

The same as the difference between version 115 and 128. The ESR tag is a prelude to moving back to a monthly release schedule like Firefox uses. With the Extended Support Release (ESR), which has been the version of Thunderbird released for most of the last 10 years going back to having that designation. See https://enterprise.thunderbird.net/explore/thunderbird-release-cycle And https://www.pcworld.com/article/2478480/thunderbird-gets-a-new-release-channel-with-monthly-feature-updates.html