A otázka zní jak? Že se liší verze Thunderbirdu?

Vysvětlení je prosté a stačí si jej přečíst na webu Thuderbidrdu:

Thunderbird version 68.1 is only offered as direct download from thunderbird.net and not as upgrade from Thunderbird version 60 or earlier. A future version 68.2 will provide updates from earlier versions. If you have installed Lightning, Thunderbird's calendar add-on, it will automatically be updated to match the new version of Thunderbird. Refer to this Calendar troubleshooting article in case of problems.

https://www.thunderbird.net/en-US/thunderbird/68.1.0/releasenotes/