Can you explain why? Apple did this two years ago and I haven't personally seen any side effects, but Safari's also not my primary browser.

It makes my life harder as a developer. I've got some widgets (hosted by me) that are embedded on my customer's websites, and because of Safari I can't use cookies for things like a shopping cart. I have no need for the cookies to be accessible from another website, even if it's got the same widget embedded, so this implementation by Firefox fits my use case perfectly. Unfortunately that doesn't change what Safari does, so it doesn't help me.

Ah! I can see how that’d be a PITA, thanks for explaining.