I think the last save as is the location... that is the install goes back to the last folder it used.

Thank you for your response Matt. As you suggest, and one would expect, the last (successful) address book export location is stored somewhere and presented as the default location for subsequent exports. Since I wrote my initial query, I have noticed that if the last export location is physically deleted then the next default location for export is randomly irrelevant. I now suspect that the issue is in Windows internals - possibly jump lists. Since the T-Bird export function clearly works by selecting a new file location this really is not a problem that needs 'solving' so I am happy to mark it solved. However, I still have an academic curiosity as to where this information is actually stored and would be interested to hear any further explanation that other contributors might have.

your will find it stored somewhere in the config editor best to search it for the path you are being prompted for rather that the preference name because I simply do not know it.

Thanks again for your interest Matt. In Config Editor I have seen two settings for last-used directories: messenger.save.dir for last saved email attachment mail.compose.attach.dir for last file attached to an email But after saving an address book to a uniquely named directory there is no trace of it when searching Config Editor. I am now just accepting this as 'the way it is' but post this info for any other searchers.

What is not in there is usually stored in session.json but it is not editable unless your a JavaScript person..