The only TLS v1.2+ cipher suites with a dedicated AEAD scheme are the ones using AES-GCM, a block cipher mode that turns AES into an authenticated cipher. From a cryptographic point of view these are preferable to non-AEAD-based cipher suites (e.g. the ones with AES-CBC) because getting authenticated encryption right is hard without using dedicated ciphers.
For CPUs without the AES-NI instruction set, constant-time AES-GCM however is slow and also hard to write and maintain. The majority of mobile phones, and mostly cheaper devices like tablets and notebooks on the market thus cannot support efficient and safe AES-GCM cipher suite implementations.
Even if we ignored all those aforementioned pitfalls we still wouldn’t want to rely on AES-GCM cipher suites as the only good ones available. We need more diversity. Having widespread support for cipher suites using a second AEAD is necessary to defend against weaknesses in AES or AES-GCM that may be discovered in the future.
ChaCha20 and Poly1305, a stream cipher and a message authentication code, were designed with fast and constant-time implementations in mind. A combination of those two algorithms yields a safe and efficient AEAD construction, called ChaCha20/Poly1305, which allows TLS with a negligible performance impact even on low-end devices.
Firefox 47 will ship with two new ECDHE/ChaCha20 cipher suites as specified in the latest draft. We are looking forward to see the adoption of these increase and will, as a next step, work on prioritizing them over AES-GCM suites on devices not supporting AES-NI.