All the variants of the payload fairing for Ariane 5 are 5.4 meters in diameter, the same as the core stage of the rocket. At the same time, the mirror of the James Webb Space Telescope is 6.5 meters. That looks like a very basic geometric problem to me. Although the mirror is modular, it is supposed to be launched in one piece, and not assembled in orbit.
There is even a diagram of the telescope nicely fitting in the fairing:
By NASA (James Webb Space Telescope in Ariane 5 fairing) [Public domain], via Wikimedia Commons
The only possibility I can see is that the payload fairing is custom made significantly wider than any earlier fairing.