In each of these cases, you still have to find another job, which means you still have to worry about paying the bills.
It also doesn't have to be your employer: it can be anyone you're financially tied to in some way.
It also doesn't have to be your employer: it can be anyone you're financially tied to in some way.
I can't give a comprehensive survey of everyone who's made FU money, but take Tim Ferris for example. He's not a whiz at building websites: he just knew to cash in on selling brain supplements online when that stuff was taking off.
Edit: I should point out that I don't think FU money is exclusively about being able to say "FU" to a jerk. I emphasized that aspect just to invert the comment I was responding to. FU money can also just be about ditching a career you're not that interested in (but may be highly skilled at, and otherwise locked into).