Can I place a form in a Popup?

Absolutely.

FireBox itself provides a Form Block which allows you to collect leads and sync your submissions with popular 3rd-party services such as MailChimp, Brevo, AcyMailing, as well as over 8,000+ apps using our Zapier integration.

However, you can use any shortcode, or Gutenberg block to embed a form from the plugin of your choice.