Ahem.
The firstborn is stripped of his diefic status because he either lost A) a record of some kind, or B) fucked with space-time via said "record" (think about: what makes you think this is a normal "record"? Or so important? somebody would know what had happened, and could make another, unless it was REALLY GODDAMN IMPORTANT/POWERFUL). This means that the firstborn may very well be the reason why you can be invaded, or interact with B.H.L., or so on and so forth. Now, losing his status could either be being kicked out of Anor Londo, becoming undead (giants in Sens Fortress), or losing alot of his power (giants sizes are based on how powerful they are, i.e. Smough getting bigger after killing Ornstein). Now, Gwyn (somehow, even though he had already departed to link the flame) decreed that his firstborn should be erased. The giants, or the WOW, whomever, the erased his name from all records, and destroyed all statues of him. Now, I doubt that the statue is of the firstborn, as, for fucks sake, they destroyed all the other instances of him, and they I find hard to believe they would either not know about those statues, or would just leave them be. I don't think it's Priscilla because A) have you SEEN Priscila?, and B) Seriously, she looks nothing like that. If it was the painted world of Ariamis only, I would believe it to be of the firstborn. But it's not. So for all we know, It could be Gwynn, It could be Havel, or Allfather Lloyd, hell, it could be Patches. But it's not Priscilla, nor the firstborn.