GPL v3 prevents mechanisms that interfere with the ability of the end user to replace the free firmware of a device, like accepting only signed firmwares. It’s an “anti-TiVo” thing. It won’t prevent proprietary drivers, but any device distributed with it must not prevent the user from making their own drivers to replace them.

There isn’t a real solution that would work for user space drivers. At least not yet. But just like GPL v3 rose from TiVo, if this becomes an issue I’m sure the lawyers of FSF will come up with something, and Linus and a lot of other folk will hate it.