Aren't firmwares for Fritzboxes provided by the Freetz-NG project?
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.
No. Fritzbox is proprietary.