You would need to export your messages from your mail provider to your computer, and then feed them from your PC into an AI tool.

How you decide which AI tool to use is something you would need to research, and have nothing to do with Thunderbird

Export your emails (e.g. via Gmail Takeout or Outlook export).

Use an email parser tool (like Clean Email or Mailparser) to extract message text.

Run a simple AI or text analysis tool to detect repeated questions or patterns.

Group and summarize the common questions.

Write clear answers and organize them into an FAQ page.

Always make sure you're handling personal data legally and ethically — especially if emails include customer info.