Their firmware isn't opensource. At least not fully. Only the EC firmware has been opensourced, that is the firmware for the micro-controller responsible for the battery, keyboard, buttons, LEDs, sensors, and so on...
The boot firmware, responsible for getting the CPU and hardware in a bootable state, is absolutely proprietary. There is initial support for one specific framework motherboard in coreboot, a partially libre boot firmware, but it's very new and I don't know exactly what is and isn't supported...