OPNsense is a fork of popular pfSense network/router/SDN product/solution.
Interesting combo here, Phalcon and Python.
the frontend implemented with PHP/Phalcon and the backend using a custom service built in Python.
As for quality, all new features will be built using a solid framework with a Model View Controller. For this purpose we choose Phalcon as it is the fastest open source PHP framework available.
So both Volt and Jinja are used, which are identical to the FE dev :)
Full source code at: https://github.com/opnsense/core