I have recently switched to the habit of checking my email only twice a day -- one hour after I come to the office and one hour before I leave. Each time I have processed any received messages I close Thunderbird. In between those occasions I sometimes want to reread a message or access an attachment and then I don't want Thunderbird to show me any new messages upon startup.
I have unchecked:
Check for new messages at startup
Check for new messages every N minutes
and (in the advanced section):
Use IDLE command if the server supports it
Still when Thunderbird starts it checks for new messages. Is there a way to disable this IMAP synchronization?