If you looking to convert your Thunderbird emails to Gmail then here is the most informative article
Export Thunderbird Emails to Gmail. This article will provide the most effective manual solution to migrate the process easily. However, the manual solution is too technical for non-technical users. Therefore, they will also introduce a professional solution
Thunderbird Email Backup Tool. The software is a full-fledged utility that exports your Thunderbird mailbox to several email clients like Gmail, Yahoo Mail, Office 365, etc. Moreover, it can save your Thunderbird mailbox to different file formats such as PST, PDF, EML, EMLX, MSG, CSV, etc. It has a simple user interface that ensures a smooth working process without any technical expertise. Additionally, it is compatible with both Windows and Mac operating systems. You can try the full-featured trial version of the software, to analyze the performance.