I would suggest you to use
Aryson EMLX Converter to convert the OLM files into Thunderbird format. This software is a risk-free & safe EMLX File Migrator utility that converts EMLX files into Outlook PST files and other formats like MBOX, PDF, MSG, DOCX, etc. It also
imports EMLX files into Thunderbird, Zoho Mail, Gmail, Office 365, G Suite and other email clients.
If you want to import EMLX files into Thunderbird then first you need to convert EMLX files into MBOX format which is compatible with Thunderbird email client. Thunderbird does not natively allow importing EMLX files. The easiest way to do this is to use the
Mailvita EMLX Converter. This application simply convert multiple EMLX files into MBOX format without deleting any attachments. You can easily import EML files into Thunderbird once the conversion is finished. The process is fast and secure and there is no requirement of technical knowledge.
Read More:
https://www.mailvita.com/converter-for-emlx/