If you are running OPNsense on a Protectli box and want to update your firmware without having to boot into a Linux distro, this is how I accomplished it:
Install flashrom in OPNsense terminal window:
pkg install flashrom
Download protectli frmware updater:
curl -LO https://github.com/protectli-root/protectli-firmware-updater/releases/download/v1.1.47/flashli.tar.gz
Extract and study the flashli/configurations.py file. It will tell you which command/parameters to use for your particular model.
Read the existing BIOS from your box and save it to a file (just for safekeeping):
flashrom -p internal -r oldbios.bin
Find the new firmware images inside the images folder and pick the one for your model (the ones starting with "protectli" are coreboot, the others are AMIBIOS).
Construct the flash command as needed. For my FW4B, it looked as follows:
flashrom -p internal -w fw4b_YLBWL440P.bin
After flashing succeeds, fully shut down and boot your box back up.