When in doubt Use the source, Luke... ;-)
https://github.com/smokku/pebble/blo...irmware.cpp#L8

Pebble App gets the latest available firmware from http://pebblefw.s3.amazonaws.com/pebble/.../latest.json so it is always up to the latest version published by Pebble company. No magic in use there. When Pebble company pushes new firmware to AWS, pebbled will have it available. No sooner, no later.

(pebbled upgrade may be required only, when a new Pebble model hits the market - pebbled needs to know how is it codenamed and where to fetch its latest.json from.)