CAPTCHA in FireHelp is powered by reCAPTCHA by google.
To setup, first visit reCAPTCHA website and login using any Google account
Add your domain on there, you will be provided with site and secret key in "key" section.
Copy these values , then head to your admin panel In "Settings" => "Recaptcha" and paste the keys you obtained
Save, then head to "General Settings" and then enable Captcha under "Authentication" tab.
Save, and test .