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Firebrick are more stable from a thermal standpoint. They are pressed, not extruded, solid not cored, and they have a different chemical composition than common brick. Common brick can take the heat on their face, but the thermal shocks of the rapid heating and cooling are what usually cause them to fail.
For the hearth particularly, firebricks are much smoother than regular bricks. For the dome, the firebricks are less likely to spall or flake after repeated heating cooling cycles.
You probably could build the dome out of regular bricks, but as Tscar said, if you are going to all that work, spend a few extra bucks on the firebrick and you will never get brick flakes on your pizza.
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.
This said, if you have no budget, and a pile of common brick, it's better to have a chipped oven than none at all. It's better to spend slim resources on insulation rather than the perfect brick.
I built my wood-oven purely out of reclaimed full clay bricks (taken from my ci. 1900 house after removing a wall inbetween my kitchen and dining room. It was more a case being broke and re-using all the debris from my house renovation and an urge to attemt to build a wood-oven on a shoestring.I've only fired mine once with any conviction (about 1/2 hour full-fire) so can't tell you if the project is a full success using normal clay bricks until I give it the full gothem city firing.