Common bricks will crack and spall in direct contact with flame. This is less a problem with fireplaces than with pizza ovens, where brick shards will end up in your pizza toppings, so it just depends on your tolerance for the look of cracked and chipped brick in your outdoor fireplace.
Here's a quote from rumford.com:
You might have noticed the red brick firebox. It's regular solid facebrick (no cores) laid in regular Portland cement mortar. They didn't have firebrick in 1760 and my customer didn't like that "ugly yellow firebrick". I told him facebrick couldn't take the thermal shock and would crack and spall, but I couldn't convince him. A year after I built the fireplace he called to tell me how much he enjoyed the fireplace and oven. He said five of the brick in the fireback had cracked and the face had spalled off another one. "It's great", he said. "The fireplace is only a year old and it already looks as if it's 200 years old." Another sort of customer would have had me back out to replace brick six times.
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