It's easy to manually import EML files into Mozilla Thunderbird. You can start Thunderbird, create a new folder in Local Folders and then drag and drop the .eml files into that folder. Thunderbird will show the emails by itself. But this method works well for only a few EML files, therefore it's not the best option to import multiple files at once. The ImportExportTools NG add-on for Thunderbird lets you import multiple EML files and folders. The Sametools EML Converter is a better approach to convert EML files into MBOX files, which Thunderbird can read. It also maintains email metadata and attachments while doing the conversion.