I'm not privy to the details, but to the best of my recollection, Mozilla's contract with T2Mobile is explicitly designed to avoid the problem of stagnant updates. Namely, they're required to "provide the the Flame with updates to each new major version of Firefox OS and a simple mechanism for switching between different release channels" for quite some time.

This is also the device that Mozilla's own employees will be standardizing on for the next year or more, so it should have a relatively long useful life.