I use Thunderbird and backup several computers E-mails at least twie a year (in fact just today)
Depending on your OS I search for the Thunderbird data folder
Under my ID on the computer I look for "app data" (A hidden file so unhide it) then "thunderbird-Profiles and after selecting the right profile (if you have more than one like I do for the wife) then the folder "mail" then the folder in "mail" that contains your most recent entries by date e.g. "in box" This confirms that I am in the correct mail program if there are more than one to choose.
I then copy this "mail" folder to my backup location. If I am copying it to another computer with an existing mail folder I usually delete that folder first so I don't duplicate unwanted files. Of course you may find it better to you interests to simply copy over an existing 'mail" folder and sort out the scrrappage later.