I have four brick posts that are about four feet high and square (1.5 bricks per side all the way up). They are going. A tree hit two of them and I was probably going to take them out anyway. So I have a bunch of bricks with some of the bricks the mortar is attached. Throw them away or use for fire pit - is it worth the effort? Also will one layer of brick be too wobbly for a rather large fire pit - 4 feet by 2.4 feet by 2-3 feet high?
My vote is always for re-use (except the trash pieces, which can still be used as a filler). For your question of using a single layer of brick (I assume you mean vertical layer for the walls) -- bad idea without lining it with steel or firebrick, and a quick way for people/things to get burned. You can also use two layers spaced with an air gap or filled with loose or crushed rock (and you can use the trash pieces as well so you don't need as much rock). You will want to mortar the bricks (mortar), not stack them, and you'd need a solid base. I'd recommend using this information from The Family Handyman as a fairly good guide.
Otherwise, build a proper firepit and use the bricks for a walkway or as a flat area for standing/seating around the firepit. I'd suggest knocking off the larger chunks of old mortar with a rock/brick chisel.