The "firewall" phenomenon was proposed in 2012 by physicists Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully[1] as a possible solution to an apparent inconsistency in black hole complementarity.
[5] According to quantum field theory in curved spacetime, a single emission of Hawking radiation involves two mutually entangled particles.
Along similar lines, Juan Maldacena and Leonard Susskind suggested in the ER=EPR proposal[16] that the outgoing and infalling particles are somehow connected by wormholes, and therefore are not independent systems.
[17][18] The fuzzball picture resolves the dilemma by replacing the 'no-hair' vacuum with a stringy quantum state, thus explicitly coupling any outgoing Hawking radiation with the formation history of the black hole.
Matter passing through the event horizon into the black hole would immediately be "burned to a crisp" by an arbitrarily hot "seething maelstrom of particles" at the firewall.
[5] In a merger of two black holes, the characteristics of a firewall (if any) may leave a mark on the outgoing gravitational radiation as "echoes" when waves bounce in the vicinity of the fuzzy event horizon.