If you're looking for an easy method, the MigrateEmails OST to PST Converter Tool is your ultimate solution. This all-in-one tool allows you to migrate single and many files in batches. The interface is really user-friendly and it will migrate with 100% accuracy and no data loss. This software is quite flexible and maintains the folder hierarchy throughout the conversion process.

Step by Step Guide to Importing OST file to Thunderbird
1. First off, as the administrator on your computer, run the tool.
2. Select the OST files, by browsing, that you want to add to the software wizard. Click Open > Next.
3. Once you see the tree structure, click through to preview and choose the OST files you want to continue. Click Next.
4. In the next window, select Thunderbird in the drop down list of file formats to save the OST.
5. Then you will have to select any filters you want to apply, before you click the Path to Save button to identify the destination (by default it sets the local Thunderbird folders for you).
6. Finally, click on the Convert button to start the migration process.

Know More: https://www.migrateemails.com/blog/how-to-import-ost-file-to-thunderbird-manually/