The link shows a list of firmwares. The actual sources are at https://github.com/Freetz-NG/freetz-ng - but it's not the original FritzBox firmware sources:

Freetz is a toolbox for developers and experienced users to build a modified firmware based on the original firmware for the DSL/LAN/WLAN/VoIP-Routers AVM Fritz!Box and T-Com Speedport (identical hardware) and to transfer this firmware to the device. There are many extension packages available, along with options to remove unwanted functionality from the original firmware.