Thanks to the hard work of Ricardo Meza and his team, the first language pack in Mexican Spanish (es-MX) is now available on addons.mozilla.org (AMO):
Español (México) Language Pack 3.0.2pre
This lang pack is still in the sandbox so you will need an account on AMO and be logged in to be able to install it.
More information (in Spanish) on the Mozilla Mexico localization portal as well as on the Mozilla Hispano news site: