The vendor must provide Hubitat with a firmware file, or otherwise they must be released publicly. The "Update Firmware" command, which I assume you are talking about, matches some identifiers from the Zigbee data (so they only get applied to the correct device). There is some more information here:
I'm not aware of Hubitat having any Sonoff firmware updates (or any having ever been released, but I can't say I've paid attention). If there is anything, you'll see something in "Logs" when you try, as mentioned above.
