Ground wires on the firewall
 

I noticed two small ground cables hanging on my firewall in the engine compartment. They aren't hooked up to anything and I'm wondering what the heck they are.

The engine block is usually grounded to the firewall by a copper strap. Not sure about the other one.

Those ground straps are for radio noise suppression, one to each (left & right) valve cover bolt, along with the wire loom bracket.
Those straps aren't the main engine ground, that comes directly from the battery.

Thanks guys. I don't get any interference on my aftermarket radio so I guess I don't need them.

You used to be able to get radio interference with the points & condenser distributor. Maybe your ignition has been upgraded?

Yes it has. That must be the reason.

Still doesn't hurt to hook each one up to a valve cover bolt.

you can never have to many grounds.those straps are to ground the body to the subframe to complete the ground circuit.because the body is mounted on rubber.

Rick's/Ecklers has an engine compartment ground wire kit that has three small ground wires. I know that two go from the firewall to the valve covers. My question is where does the third one go?

Between the pass side subframe connection and subframe.
Looks like this: