You can pass on parameters in the URL towards Firmhouse Checkout to pre-fill these fields.
For example the following URL would prefill the customer’s email address:
https://checkout.firmhouse.com/en/projectslug?subscription[email]=michiel@firmhouse.com
Pass in multiple parameters:
https://checkout.firmhouse.com/en/projectslug?subscription[email]=michiel@firmhouse.com&subscription[name]=Michiel
You can check the Checkout page source in your browser to see how the fields are named. Just add the name of the field as a URL parameter and you can pre-fill it that way.