If you have a Pop mail account, Thunderbird can only access the Inbox on the server and download anything that has not been previously downloaded. This is how all pop mail accounts work. If you have an IMAP mail account you need to '''subscribe''' to remotely view server folders. You subscribe a folder by right-clicking on the remote Inbox, selecting "Subscribe", expanding the Inbox tree in the window, and subscribing whatever folders you want. You can configure it to show all folders by unchecking the checkmark at "Tools -> Account Settings ->Server Settings -> Advanced -> IMAP -> Show only subscribed folders". You can also choose to synchronize folders for offline use. More info on IMAP mail accounts: * https://support.mozilla.org/en-US/kb/imap-synchronization?esab=a&s=imap+synchronise&r=0&as=s