If you want to convert your Thunderbird to new computers then you will go with this informative article Transferring Thunderbird To New Computer. Here in this article, you will get the most secure two approaches. 

If you want to convert with this method, you can perform your transfer task in two methods.

1. Create a backup of the Thunderbird Data Locally. 
2. Converting Thunderbird profile to a New Computer. 3

However, In this article, they will also provide a professional solution Thunderbird Backup Tool. Which helps to easily backup and convert Thunderbird mailbox to several files or folders and different email clients. You can also split your large resultant PST file into small parts through the Split Resultant PST feature. To explore the performance you can download it.