I agree with
@rninja that Monevator is pretty good and their series on making calculations on retirement etc
here. Also Banker on Fire has a couple of interesting bits
here.
IMO, FIRE is similar to lots of other crazes etc. online. The loose principles behind it I fully support - more planning, not getting into lots of unsustainable debt, putting enough money into pensions for later in life etc. However, like all these things, there are people who always take it to extremes and try to live in a car or whatever to save as much money as possible and miss out on life to then live miserably ever after on the amount of money they have saved.
In-between these two things is a happy medium! I wouldn't say I am following FIRE, but I do make sure I properly budget (whilst taking holidays and doing what I want to achieve in life generally) and have some money going to pensions etc in the most tax efficient way. I don't have any solid plans for the future, but at least having some pension savings is buying me options/flexibility later in life.