You can be FIRE at around a million in assets. My wife and I ran the numbers and it seemed possible. But we wanted kids and stuff so retirement wasn't in the cards.

If you have FU money you don't need to run the numbers.

That's how I look at it.

Whether or not there's a formal difference in the terms, there's normally a difference in how they're used.

1. FIRE tend to mean sufficient money in the bank/money from passive incomes that let you live a fairly frugal lifestyle. See the Mr. Money Mustache stuff that various people are especially taken with. It doesn't tend to mean Retire Early and take expensive trips, buy nice cars, pursue expensive hobbies, buy second homes

2. FU money, on the other hand, tends to mean you don't need to work and can have enough money to do virtually anything you want to. There are degrees of course once you're talking things like private jet ownership.

Not sure about the context of the sibling posts (FU money might mean something different in SV) but within the FIRE community (UK atleast) FU money tends to refer to less money than full FIRE. FIRE means you never have to work again (normally 25x annual spending), FU money means you can quit in extreme circumstances and have enough money to tide over any job search (min of 6-12 months spending).