This is a perennial question for those having troubles with Thunderbird. It is a fairly simple topic on the face of it, but becomes complex once you scratch the surface.
If we assume that a mail account is a pop account, and the default settings have not been changed then Thunderbird will be making a permanent store of your mail locally.
Thunderbird stores all of your account settings and data as well as add-ons and themes in your profile folder. The profile folder when opened in Explorer will always contain a Mail folder, even if all your mail accounts are IMAP. Mail will exist and contain "Local Folders".
The mail folder contains folders named after the mail servers you retrieve mail from., as the following image shows
The image below shows some of the files and folders in my "Local Folders" folder in Windows explorer
Looking at that image you will notice that there are three entries for Archives.
|Archives||This is your actual stored mail. This file may be very large,
although if it exceeds 4Gb you might run into troubles as the hard
limit of 4Gb is only recently removed.
This file is stored in the Berkley MBOX Sub Type RD file format.
|Archives.msf||A glorified index, which some of the development community
refer to as a database. It is the information Thunderbird uses to
actually populate the lists of mail you see in Thunderbird. But it contains no information other that tag that can not be recovered if the file is deleted.
This file is stored in the MORK file format.
|Archives.sdb.||This is simply a file system folder. It exists to hold the files contained in the sub folders of Archive|
This twin or triple named sets occurs throughout the mail folder, with sdb folders only occuring when there are sub folders of the folder in Thunderbird.