To Import EML Data to Thunderbird manually, you need to create a new folder within Thunderbird, right-click on it, and select the option to Import all messages from a directory > also from its subdirectories. Then, browse and select the folder containing your EML files. Thunderbird will import them into the selected folder. However, the manual method comes with several challenges: it requires technical steps that may confuse non-technical users, doesn't support bulk conversion efficiently, and can lead to data integrity issues such as missing attachments or altered metadata. 

To overcome these limitations, using a professional solution like the Cigati EML Converter Tool is highly recommended. This tool allows users to import multiple EML files directly into Thunderbird without needing additional add-ons or risking data corruption. It supports batch conversion, preserves folder hierarchy, and offers features like previewing emails before conversion and filtering data by date. Moreover, it simplifies the process for non-technical users with its intuitive interface. Hence, for a secure, efficient, and hassle-free way to import EML files into Thunderbird, Cigati EML Converter Tool proves to be a more reliable and user-friendly option.