Thunderbird Backup Tool
is a full-fledged utility that saves your Thunderbird mailbox to PST, PDF, EML, EMLX, MSG, CSV, TIFF, etc. It also migrates MBOX files or folders to several email clients like Gmail, Yahoo Mail, Hotmail, Office 365, IMAP, Zoho Mail, etc. You can split your large resultant PST file into small parts through the split resultant PST feature. You can download the free demo version of the software to analyze the performance.