I was sure that I could just connect via ethernet and install the Firmware app and then install the Broadcom firmware from there. But no, Firmware can not see the Broadcom chip!
But thankfully I found a guide on cyberciti.biz... Yeah, that sounds sketchy, but it worked! But it was a bit long, and Firmware still doesn't see the chip even now! So this is not ideal. Why is it so hard to install?