yep you should do that if you were s-off. while s-on you can't downgrade.
firmware would be the same as bios... that's the idea. but in fact it is much more.
a firm comes from otas which typically contains those partitions:
the firms posted around contains the two first partitions.
what you could try is flash the stock recovery, and a stock boot & system image and wait to see if an ota is coming.
EDIT : or while stock pay 25$ to s-off. and then you could flash whatever firm you want. by the way you seem to only need a radio.img (this is one which is invalid
but where can I get the radio.img and OpenDSP.img alone. Also, I didn't quite understand the following "or while stock pay 25$ to s-off."