This is not really a standard inheritance story, but for what it's worth... I ended up being an heir to a completely unexpected inheritance - ironically, from a close family member who stole my entire savings, which I had set aside to cover university costs... There is another potential heir - a beneficiary of the theft, and at least a post facto party to it (I'm unclear whether they knew exactly what was happening at the time) - who is currently engaging in legal actions that are burning through the estate. If that person suffers from a sudden and unprecedented attack of rationality, I will potentially end up inheriting a meaningful sum, as would they. If the legal stoush continues, both of us would potentially inherit nothing - which is simply status quo for me, but would likely render them destitute: this inheritance was their sole financial plan. In other circumstances, I might have more sympathy. In this case, not so much...
I am extremely happy that I am not in a situation where this is money I "need" - or even money that would make a difference in our plans. I am very happy that I can just hand things over to the lawyers, and really not have to care how things turn out. These people have, at earlier stages of my life, done enough damage: not this time, and that's a really good feeling.
I do loosely have some ideas what I might do with the money - but those ideas are deliberately ringfenced from our FIRE plans. This is partially rational - due to the uncertainty - but it's also partially due to idiosyncratic emotional motives: I worked extremely hard to get where I am, in spite of these people, and I don't want any uncertainty over whether I could have gotten here without their help. I'm particularly happy that we have enough for a thin FIRE now (we're secure enough that we downshifted this year - I took a substantial pay cut to move to a much more pleasant working environment, in the location we want to retire - and, in one of those ironies that often happens in the FIRE community, we've found that we're actually saving slightly more than we were last year, even after the drastic drop in pay, because of the extra amenities in our new location and the extra time freed up by the new role...).
While my situation is weird, I suspect there will be others who might feel a bit of a letdown if something happened suddenly to bring them to their FIRE target. I'm not saying people /should/ feel disappointed: all of us capable of FIRE benefit from luck on many levels, and inheritances can be well or badly managed, just like all other kinds of luck. But it wouldn't surprise me if some people who had really worked hard to optimise their lives for FIRE, were a bit ambivalent about a sudden inheritance.