If this is an IMAP account, your efforts may prove fruitless, as IMAP is designed to be a living, continuous, account. You can uncheck 'check for new messages at startup', but that may not do it.

Settings/General/Network & Disk Space, Offline..., set TB to start in Offline mode, then toggle the status by clicking the icon at the left end of the Status Bar.

I have one IMAP account and three POP3 accounts. The POP3 accounts were unchecked. I unchecked the IMAP account, but that didn't correct the problem. I also tried the offline option, but that didn't seem to solve the problem. I guess I'll just have to live with it. It's not vital, just an annoyance. I must be a control freak. At least I got some education from the exercise. Thanks for putting up with me. Thunderbird forever!

We are always glad to assist. Enjoy!  :)

Have you found that starting in Offline mode still results in automatic downloading of mail?

When I launch Thunderbird it's now in offline mode. I normally use alt-FW to check all accounts bat bonce. I get a question box letting me know its offline. After saying OK Thunderbird from that pointm on downloadsm all new email automatically. I'm wondering if it's a Windows 11 issue or this Dell computer. I'm doing OK. Thanks for checking.

If TB starts in Offline mode, and you manually check for new mail, TB goes online after OK, as you found. It will remain online unless you manually go Offline by clicking the icon or File/Offline/Work Offline. In other words, it's necessary to go online, check for mail, then go offline, each time you wish to get mail.