I was under the impression that "co-founder" carried a lot more weight than CTO; is this not the case? My gut feeling was that, if I were in the co-founder's shoes, I'd fire the CTO immediately.
In a small team, infighting is a much bigger threat to survival than losing skills/results of any single individual.
Depends on the circumstances. If one founder controls a majority share, they can fire anyone they wish.
In this current situation, if the CEO controls more than 50% of the shares and the CTO reports directly to the CTO, the CTO can fire the OP.