I don't think I'd make that security tradeoff. From best to worst, I'd rank the options is this order:

(1) signed open source

(2) unsigned open source

(3) signed proprietary

(4) unsigned proprietary

In some cases I might rank #3 as the worst option if it prevents me from editing the firmware binary (to fix a security bug myself or to remove tracking info like a serial number), reverse engineering the firmware to verify it (because signed code is often encrypted as well), or installing alternate firmware that might be more secure.