This would be dumb though, because it would tank your company.

Leaving the situation as-is would tank that company.

In a small team, infighting is a much bigger threat to survival than losing skills/results of any single individual.

Hmm, I'm sure infighting is bad, but often enough individuals do have skills and in particular experience with the setup and/or code and/or business contacts that the startup has made that you're not going to find a replacement nor train one without losing lots of time and money. Certainly that's an avoidable risk with effort, but I'm pretty sure that some level of infighting may be less risky than losing certain individuals.

You are assuming that the CTO is the only CTO in the world.

A CTO with polarized vision would tank the company faster.

So, if that would tank the company noone gets nothing. If he doesn't fire the CTO, he gets nothing and company may or may not get anything. To me, pretty clear situation what would I do. I would risk the company to save myself from certain loose.

> I was under the impression that "co-founder" carried a lot more weight than CTO

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.