The scenario motivating question #3 above is one in which a laptop is not used very often, and you fire it up to check one folder in one account. You don't want to wait forever as TB synchronizes all the changes since the last time the laptop was used. You just want to update one folder.

Of course, if answer to question #1 is that only headers are synchronized, then I suppose that question #3 is not so crucial. It just feels that way when the sync is being done across all folders. Even if only headers are synced, I think there is still a great benefit from only doing so when entering a folder of interest, especially if one has many accounts with many folders.

Thunderbird will download headers in all cases to build the local index so it can show what mail is in the folder.

If you turn off the option in account settings > synchronization and storage to keep messages in all folders for this account on this computer then unless you set individual folder options in advanced, only headers for the account will be downloaded. Turn it on and you get the whole message

I am unaware of a setting to not download attachments, but that is mostly I would say because attachments do not exist in a message as more that a stream of text until the message body can be parsed to determine if there are attachments and which mime parts contain them. It would not be possible to determine what parts of the body are attachments without actually accessing the body ( downloading it) to work it out.

You might want to look at the settings in account settings > server settings to disable the settings under Server settings. All three of them with them disabled and the local storage turned off you will basically get only updates on selecting the relevant folder.

Thank you, Matt. I'm going to hold off exploring for now because I've been disconnected for a while and I don't want to absorb the overhead of syncing at the moment. But I understand your explanation and believe that it fills the gap in my knowledge.