They are basically similar. 128 is an extended support release, with major updates annually, while 138 is a monthly update that always has the latest features. More info here:

https://blog.thunderbird.net/2025/03/thunderbird-release-channel-update/

1. Version 128.10.2esr belongs to Extended Support Release (ESR) Channel. Version 138.0.2 belongs to Release Channel.

2. ESR Channel upgrades once per year. Release Channel upgrades once every four weeks.

3. If you are using add-ons, or if you think you will use add-ons in the future, or if you are not sure whether you will use add-ons in the future or not, I suggest you use ESR Channel. If you are not using any add-on, and you are sure that you will not use add-ons in the future, you can use Release Channel.

Reason: It is a common situation that after upgrading Thunderbird, the add-ons you use become incompatible with Thunderbird. ESR Channel upgrades once per year. Therefore, if you use ESR Channel, the add-ons might become incompatible with Thunderbird once per year. Release Channel upgrades once every four weeks. Therefore, if you use Release Channel, the add-ons might become incompatible with Thunderbird once every four weeks.

4. References: (1) Choosing a Thunderbird Release Channel https://support.mozilla.org/en-US/kb/choosing-thunderbird-release-channel (2) Thunderbird Release https://support.mozilla.org/en-US/kb/thunderbird-release