Normal firebricks wouldn't withstand mid or high fire temperatures. They'd crack or melt or both. An iron grate wouldn't fare well either. You could probably make modifications to the fireplace that could make it possible, but that that point you'd probably decide to take the whole thing away from your wooden floor/carpets.
DANGER: Most home fireplaces are not designed/constructed/maintained for longer-duration fires of any serious intensity. If you try to use one as a furnace or kiln, you can easily burn down your house.
I tried this with some cheap "low-fire" clay that bought online and wanted to make into pots. I tried firing them in my metal "stove" style fireplace (these are the iron looking ones that are fairly common at least here in the UK) The first batch simply exploded in the fire as the clay was not dry enough (sounded like I was making popcorn). To thoroughly dry the next batch of pots I let them air dry for a week and then 'cooked' them in the oven for a few hours. I placed them in the fire, and kept it burning for a good few hours feeding in wood fairly consistently, although the temp was nowhere near hot enough for a 'proper' firing.
It 'sort of' works - the pots are very brittle and not at all waterproof. Glazing wont work at those temps so I tried a slip-glaze (basically glazing with liquid clay) which at least gave the pots a slightly shiny appearance.