First, you need to export your emails from Opera in MBOX format to move Opera Mail data to Thunderbird manually. Open Thunderbird and go to Add-ons, where you will add the plugin "ImportExportTools NG". After addition, go to the import options, select "Import MBOX File", and select the MBOX file you saved from Opera. It will then import the mailbox and show the emails inside the selected folder within Thunderbird.

The disadvantages of this manual approach are possible file errors, incomplete imports, and possible choking on large MBOX files. Additionally, there are extra steps required, such as the installation of add-ons, which may not be very friendly to non-technical people. Bulk migrations can be slower, and attachments or formatting may not always transfer perfectly.

MacSonik MBOX Converter Tool is a secure and risk-free utility that exports Opera Mail to Thunderbird without data loss. The tool is capable of transferring multiple Opera Mailbox folder data into Thunderbird email clients. The usage of this software doesn't require any prior technical knowledge. Naive users can easily handle this software because of its interactive and easy-to-use GUI.

Step-by-Step Guide to Migrate Opera Mail to Thunderbird
1. Download and install the MacSonik MBOX Converter Tool. Afterward, run it as administrator.
2. The software displays the folders of the Opera Mail account.
3. Now, check and select the folders from the tree structure which you want to migrate.
4. Here, you can also preview mail and their attachment files and click Next.
5. In the drop-down list of Save/Backup/Migrate As options, choose Thunderbird.
6. Click the Path To Save button to choose the destination folder.
7. Now click on the Convert button to initiate the migration of Opera emails into Thunderbird.
8. You can click the Download Report button to save the report of migration.

Know More: https://www.macsonik.com/blog/migrate-opera-mail-to-thunderbird/