I've just been doing some testing (using bleeding-edge not-yet-public packages), and got my Pi into a "slightly odd" situation. I had a chat about it with @XECDesign and he says that the instructions at https://www.raspberrypi.com/documentation/computers/os.html#how-to-get-back-to-safety should probably be trimmed down to just

sudo apt-get update
sudo apt install --reinstall raspi-firmware

(apparently the libraspberrypi* packages are being deprecated), but he also said that it's worth double-checking with @popcornmix