Yes, you can migrate only selected Thunderbird folders to Yahoo Mail instead of moving your entire mailbox. If you do it manually using IMAP, you would need to drag and drop the specific folders from Thunderbird to your Yahoo Mail account, but this can be slow and may risk losing attachments or folder structure. A much easier and safer way is to use a tool like the CloudMigration Thunderbird Backup Tool. It lets you choose exactly which folders or emails to migrate, preserves all attachments and metadata, and moves them securely into Yahoo Mail without affecting other folders.

Here are the Simple Steps to Migrate Thunderbird to Yahoo Mail:
1. Open the CloudMigration Thunderbird Backup Tool on your computer.
2. Select your Thunderbird profile or mailbox.
3. Choose the specific folders you want to migrate.
4. Pick Yahoo Mail as the destination and sign in to your account.
5. Click Convert to start the migration.

Your selected folders will appear in Yahoo Mail safely.

Read More: https://www.cloudemailmigration.com/blog/migrate-thunderbird-to-yahoo-mail/