Define Oro Contact 'firstName' field as filter and sorter in OroCRM REST JSON API.
Originated from:
$ composer require pedrofurtado/orocrm-firstname-api-bundle
After installation of this Oro Bundle, execute in root folder of oro project:
$ php app/console oro:api:cache:clear
$ php app/console oro:api:doc:cache:clear
$ php app/console cache:clear
$ php app/console oro:migration:load --force --show-queries
Then, restart the application, clear all caches, and enjoy it!