Import OST to Thunderbird – Directly It is not possible to import. But you can convert the OST file to Thunderbird supported file format. Here is the details topic on OST/PST and Mozilla Thunderbird.
The OST & PST files
OST and PST are two variants of the Windows Outlook file. OST is for offline storage and PST is for personal storage. You can choose in Outlook settings what file you want to use for downloading and storing the emails at. But by default, anytime you add an IMAP or Exchange account to Outlook, all the emails while syncing from the mail servers are downloaded to OST file.
This lets you access data even offline. You can also make changes in an offline mode and even send an email (which will be kept in Outbox folder). When the connection to internet is back, Outlook automatically synchronize the changes with the mail servers. This keeps your database same everywhere.
PST is used when you add a POP account, which is relatively an older protocol for accessing and downloading mails from the servers. With a POP enabled email account and PST file, the emails are simply downloaded to your local machine for you to read them. But it doesn’t change anything on the servers. If you delete an email, it will remain on the servers because there are no active synchronization in place. (though, many email services allow deleting the email once the client like Outlook has requested for downloading an email).
Many do not know about this mechanism and they often don’t need to.
However, the trouble starts brewing when there is OST file and nothing else. Because that is impossible to import OST to Outlook or open with Outlook (or any other client) without the original email account it was associated with.
That is just the nature of OST file. It is not meant to be used manually for tasks like importing/exporting, archiving, migrating, or doing anything with the data. The file format works internally and automatically with Outlook for the seamless functioning with the client and the servers.
But any how you have the OST files you with and you want to import or migrate it to Thunderbird. Such projects of import OST to Thunderbird can be frustrating, considering the file is not even import-able with the Outlook.
But, here is what you need to hear:
It is possible to import OST to Thunderbird, though, not directly. By converting it to the format that Thunderbird can detect and import, you can move the data in OST to the client.
You can also use OST with Outlook (both Windows and Mac) if you convert it to PST.
Tool to Import OST to Thunderbird
OST Extractor Pro is the app that will do this for you. It works amazingly well and accurate with all kinds of files and databases and details they might contain. Many common solutions readily available online have trouble getting the information converted while maintaining the fidelity. They output many errors and omitted components that could be crucial for the users (like attachments, headers, inline images and graphics, and so on).

This app, OST Extractor Pro, is designed with the necessary power and functions that don’t result in any sort of errors. The precise algorithms work towards complete and thorough conversion and also maintains the structure (like folders hierarchy, nested emails hierarchy, text formatting, and so on).

Try it to import OST to Thunderbird
Pick up the trial version if you are curious to find out more. The app features many other options and output choices than just import OST to Thunderbird. You can also use for converting OST to PST (If you want the file to be used with Outlook), Apple Mail, MBOX, EML, and other formats.