The mobile app doesn't support direct import or transfer of emails from TB desktop. They can only be downloaded via your mail providers server. If your TB desktop account connects via imap, your emails will exist on the server and auto sync to the mobile app. If the connection used pop, your locally stored emails will need to be uploaded to the server first.

Thanks for that.

May I ask a follow-up question please.

Is it possible to create personal folders in the Android App?

The Android app does not currently support folders creation.

If your account connects via Imap, a workaround is to create folders using your webmail. They will then sync to the mobile app. Thereafter you can move emails to and from these folders within the app.

Support for Imap folder creation has been requested on Mozilla Connect linked below. You can up-vote this or raise your own ideas. https://connect.mozilla.org/t5/ideas/add-the-ability-to-create-imap-folders/idi-p/72777