The answer to that may be to stop using the web browser to do damn near everything. Your computer will load an app, distributed by a trusted app store or downloaded directly from the device, that talks only to the device on the local network, and to a whitelist of allowed internet hosts, and nothing else. The app will have a client certificate so the device won't blindly trust the computer either. Firefox will only communicate over the local network in a limited way, enough to download the app from the device or get a link to the manufacturer's app store profile. Or that discovery can become part of the operating system and browsers will stop talking over the local network at all.