I'm not sure I'm picturing this correctly, but if embers are falling down into the chimney below the fireplace, the danger is small as long as the cleanout door in the basement is tightly closed. If it is not, you certainly don't want live embers spilling out onto the basement floor.

On the other hand, crumbling mortar in the firebox may indicate other problems such as water entering the chimney, a loose damper, etc. It would probably be worth while to have this checked by a mason or chimney sweep, if there are any of the latter in S. CA.

Thanks for the tips. I keep the door shut tightly as you say. It's is all new to me, I'm just used to the brick floor.

Thanks again.