Does it need to be some infected firmware USB stick that you bought from a 3rd party or can it be a regular brand new USB Stick from a retail store which gets infected if your computer has some virus.
There was one guy who had like 4 CDROMs and he burned the unsigned transaction on a CD-Writer, put in the CD-Reader on the offline computer and burned the signed transaction with the CD-Writer, and later on with the online computer put it back into the CD-Reader to get the transaction broadcasted. This seems a little tedious to me.
Any safer methods on getting the transactions signed or is this "BadUSB" just something that rarely happens with Bitcoin?
i have never heard of bad USB which has a malicious firmware! if you have any link i would love to read more about it thought.
and as for that method i think it is an overkill and he is not really doing anything extra to make it safer. the best way to have an "air tight" system for signing transactions is an offline linux which you never attach anything to it. and for the transactions you only use QR codes and your camera to scan them.
check this out: https://susestudio.com/a/kp8B3G/ciyam-safe
her is the person who made it: https://bitcointalksearch.org/user/ciyam-44572