This isn’t really anything to do with the ZigBee binding. The firmware provider is completely independent, and the OTA protocol itself is already supported in the binding.
I believe in the past the plan was to have such a provider, but that was in ESH so I’m not sure what the status is now. I’m also not sure where the documentation is - I think there was an issue raised to add documentation as there was/is none in ESH, but I’m not sure.
I have implemented a provider, but unfortunately I can’t provide that publicly at the moment - maybe in future I will find some time to sort it out.