Yes and its very easy actually.

1. Create a folder on your computer to save the files to. Like “email-archive” or something.
2. Open Thunderbird
3. Select the emails you want to save (If you want to save an entire folder, like the Inbox folder for example, just press Ctrl + A to select all emails in that folder)
4. Right click anywhere in the list of selected emails
5. Select "Save As…"
6. Select the previously created folder in the save dialogue
7. Done. You now have thousands of .eml files :)

They are just specially formatted text files that can be opened with email programs or text editors. This means you can easily do a full file content text search through the entire folder if you need to find something.