Interesting theory, but I dont't know if it is really because of the gruesome death of Branson. It surely had an impact on Daniels, but not as much that she would be afraid of fire from that point on.
Of course it could just trigger a heavy emotional response instead of a "Pyrophobia" (I just made that word up).
One thing I want to point out is that we actually never really *knew* that they were afraid of fire. In Alien Ash just made an assumption, due to the fact that most if not all animals are afraid of fire. Maybe he even lied purposeley, knowing that it would do absolutely nothing. As a way of protecting his discovery.
If I recall right the Marines kill some Aliens in Aliens. (pun not intended)
But are they afraid? They were also shot with the pulse rifles, but they were surely not afraid of them.
Ripley says the Alien was afraid of fire. It was her assumption, maybe her only hope. But this was never really put to the test.
