raspberrypi-firmware package seems to upgrade kernel (this is not expected at all. The package description says that it’s temporarily, though. I guess we’re at the Raspberry Pi Foundation’s mercy at the moment for a while).
We’ll see what can be do about it, though.
If you cannot wait, there’s a pretty simple hack. You need to backup a couple of files and overwrite the files that are getting modified during the upgrade
mkdir -p ~/backup
cp kernel7.img kernel.img bcm2709-rpi-2-b.dtb ~/backup/
sudo mv ~/backup/* /boot
Something like this would do.