Thunderbird should have the ability to disable ALL telemetry and other unknown connections, in fact this should be the default. The only place Thunderbird should be connecting to is the email provider's POP and SMTP servers, and/or IMAP servers. If it wants to connect to anything else there should be a way to get a full explanation of where it is going, why it is going there, and we should have the ability to block that connection if it is doing things we don't want it to do. Some of us do NOT want it trying to sync anything to any online cloud service!
A connection that ONLY checks for updates to Thunderbird and does nothing else that would be fine too, but as it is Thunderbird just wants to connect to various thunderbird.net, mozilla.com, and firefox.com addresses with no explanation of why or what it's doing.
Thunderbird needs to be much more transparent about what it's connecting to, and give users the opportunity to opt out without having to use external connection blockers such as OpenSnitch (Linux) or Little Snitch (MacOS). The "just trust us" mentality that many developers have just doesn't cut it in these times, particularly for anyone who values privacy.