Your question puzzled me, as what you are requesting is normally the default. Is it possible you use Gmail? If so, settings for that are in the Gmail website. On Thunderbird, right-click the account in left pane and a panel should appear with the 'subscribe' option. It will show all folders that you're eligible to display. If you use Gmail, then you must first define the folders as being visible in Gmail website. I may have misunderstood your intent, but I do hope this is helpful to you.