Having the FireBrick manage a Letsencrypt certificate is easy and works with little config - set the hostname and email address.
You are not restricted to Lets Encrypt though, you can use Buypass's ACME system to obtain a certificate.
Buypass Go SSL
Buypass offer a free certificate under their 'Go SSL' brand.
You'll need to manually install the Buypass Class 3 Root certificate first. Download and install Root (Buypass Class 3 Root CA) from the Buypass website.
Then set the ACME url to be https://api.buypass.com/acme/directory
Other providers offer free certificates via acme, but require 'EAB credentials' which the FireBrick does not support at this time.