Another option, for those sites which rely on third-party cookies to work (they're rare, but they do exist; so far, I've only found one), would be to scope the third-party cookies. That is, if both foo.com and bar.com do a request to thirdparty.com, each would have their own set of cookies for thirdparty.com, and both would be different from the cookies used when thirdparty.com is accessed directly.

Does the Firefox option "Accept third-party cookies: Never" mean that origin server is disallowed from setting 3rd-party cookies OR that website-initiated fetches to 3rd-party origins don't include cookies ..or both?

You mean those vendors that make money using third party cookies to track users?