if you were talking about devices that could be updated remotely without any user intervention, then there could be a valid argument about the 'equality' of the owner with the device manufacturer.
But since almost no devices work that way, and if they have flash, frequently require specific user actions to update the system (booting into special modes for example), usually along with big nasty warnings about how dangerous the upgrade is. It seems very clear that the power to upgrade or not is already in the hands of the owner of the device. nobody forces them to upgrade.
the few devices that do have remote update processes (Sony game consoles for example) make no pretension of being free or open in any way and are not harmed by the FSF position
the fact that the FSF has actively encouraged firmware to be burned into ROM so that it could not be modified by anyone as an "improvement" over the exact same binary blob being loaded by the otherwise free OS is a clear indication of how wrongheaded they are. They would rather have a device that the owner cannot control that runs a binary blob instead of a device differing only in that the binary blob gets loaded to the device by the free OS instead of by additional hardware.