Does anybody here know when Firefox (and other major browsers besides IE) dropped the P3P standard? I certainly recall having to deal with this on most (all?)

Does anybody here know when Firefox (and other major browsers besides IE) dropped the P3P standard? I certainly recall having to deal with this on most (all?) popular browsers ~6 years ago. I'm curious if they silently dropped it (because it does make the internet more useful, but more dangerous), or if they actually gave rationale for discontinuing its use.