There are many free techniques to import Windows Live Mail files into Thunderbird, but they have certain restrictions. So, I would suggest you try
DailySoft EML to MBOX Converter Tool. This tool helps to convert EML files to MBOX format which is compatible with Thunderbird email client. With this tool, you can convert bulk EML files into MBOX. It has a user-friendly interface, so past technical knowledge is not required. It easily runs on Windows 11, 10, 8.1, 8, 7, XP, Vista, and below versions.
visit here:
https://www.dailysoft.org/eml/mbox/