Leads to a prisoner's dilemma situation. A move like that has to be done by everyone in concert (example: killing Flash), or it's harmful to the one browser that blinks first.

This thread contains plenty of examples of legitimate uses for third-party cookies. If FF instantly and immediately broke those, users would be cursing, not praising Firefox, and switching to a browser that doesn't break what they use.

Can't we whitelist some of third-party cookies for the transition period?

If they were going away for real across all browsers, yes.

Historically getting some browsers on board with that program has been very difficult.

Concretely: a large fraction of website developers would much rather put up "only works in Chrome" notices than fix their crap.

[Disclaimer: I used to work at Mozilla, and have done my share of trying to push for turning off third-party cookies.]