In any modestly recent construction, fire alarms should have AC power behind them, allowing the Nest Protects to set up a proper mesh network amongst themselves, whether or not they also mesh with the Nest Thermostat. In my house, I've got good WiFi coverage everywhere, but in houses without that but with enough fire alarm density, the mesh network should do the job.

Curious thought: these Nest Protects have all the necessary hardware to become WiFi APs, possibly spreading service throughout a house using the mesh network as a backhaul.