With Synology's DSM, at least, there's no "firmware" per se; it's just a regular Linux install that you have sudo(1) privileges on, so you can just SSH in and m

With Synology's DSM, at least, there's no "firmware" per se; it's just a regular Linux install that you have sudo(1) privileges on, so you can just SSH in and modify the OS as you please (e.g. removing/disabling the update service.)