What bits are proprietary is typically upto the phone manufacturer/carrier. Sometimes some portions of the bootloader is open, sometimes it is closed. Sometimes the bootloader is locked (i.e. requires a signed bootloader) and sometimes it isn't. It may also depend on which bootloader is being referred to (as there are typically several stages of bootloading). I'm not aware of any phone manufacturer which makes the modem firmware source code available. Mozilla delivers source code to the phone manufacturer and/or carrier for the gecko/gaia bits and they build the rest of the system. So the exact makeup of what goes into the phone is upto the phone manufacturer/carrier.

What bits are proprietary is typically upto the phone manufacturer/carrier. Sometimes some portions of the bootloader is open, sometimes it is closed. Sometimes the bootloader is locked (i.e. requires a signed bootloader) and sometimes it isn't. It may also depend on which bootloader is being referred to (as there are typically several stages of bootloading). I'm not aware of any phone manufacturer which makes the modem firmware source code available. Mozilla delivers source code to the phone manufacturer and/or carrier for the gecko/gaia bits and they build the rest of the system. So the exact makeup of what goes into the phone is upto the phone manufacturer/carrier.