See there are multiple ways via which you can import emails in Thunderbird. It all depend upon the source. Let me explain
For Eg: You have emails stored on an IMAP server - you can import them into Thunderbird by simply connecting the IMAP account via the "Add Account" option under the menu.
On the other hand if you want to Import emails from files such as EML, MBOX etc. like that you will need some extra support.
MBOX can be imported directly in the local folder without any extension while adding in Mail Folder under the profile and also for people with low technical skills once can - also use ImportExportTools NG. For EML files you will additionally need ImportExportTools NG there is no other way - except a very long walk around using and email client that supports both. Although it is great it is limited to only 4GB of import.
And also additionally one can also use Third Party Thunderbird Import Tool eg: By BitRecover that specialise in dealing with bulk data with integrity