Fptp is good for the left-wing half of the fptp system

All forms of democracy are conservative reactions to the desired tyranny of an authoritarian group (democracy is defensive). Democracy is always a defensive force for good and so we should extend its reach wherever possible. Democracy is conservative. If there are elections we can assume people do not want socialism they want to be left alone to the greatest extent possible. They only want socialism when they think no other option is possible or reasonable. People do not want to be looked after by the government.

People do not want socialism (when they can see it clearly for what it is) but even if that is what they want fptp offers no protection from it because socialist votes will already be lost to the socialist party. If one of the main parties is socialist fptp offers no protection. Both fptp parties would need to reject socialism for fptp to be an effective defence against it.

Fptp is a protection against socialism only if i) the electorate want socialism and ii) both of the leading parties are against socialism. If one or both of these conditions is false then fptp is detrimental to freedom. For fptp to be liberal would require both of the main parties to be to the right of the electorate. If only one of the main parties is to the right of the electorate this is not enough because the (presumed) left-wing electorate would be comfortable voting for the other party.

To be an advocate of fptp (and a libertarian) assumes both of the main parties are liberal and to the 'right' of the electorate.

First past the post is not a problem for and is tolerated by liberal people who think (not just one but) both of the main parties are anti-socialists (to the right of the electorate).