I believe Mozilla said it best here:

blog.mozilla.org/…/improving-privacy-without-brea…

Firefox’s privacy protections must be usable on the web, or people will simply stop using Firefox altogether.

The web is not at the stage yet where third-party cookies can be disabled entirely. Chrome’s phase out of them this year should push all those sites still clinging to them to fix their sites. This should mean less problems when using Firefox’s privacy features. Firefox won’t necessarily need to remove the feature soon anyways since it already isolates them per site.