First of all you need to export Thunderbird emails to MBOX file. After that downlaod and install Advik MBOX to PDF Converter. Now follow the below steps to convert thunderbird mbox to pdf files;

1. Launch tool and add mbox file
2. Choose output target location
3. Click convert

Done! No other method can beat this one. The tool will quickly convert MBOX to PDF format without leaving any drawbacks.
Highlight features of this application

• Single user interface
• File naming option
• Support Multiple language
• Convert mbox to pdf with attachments
• Preserve key elements