How many bad market years in a row can a FIRE portfolio realistically survive?

There is no single number, because survival depends on your withdrawal rate, how much spending you can cut, and what you hold in safe assets. History shows retirees have weathered multi-year declines like 2000-2002 and 2008 when they had two things: a cash-and-bond buffer to avoid selling stocks at the bottom, and the willingness to trim discretionary spending. A retiree drawing 3.25-3.5% with a couple of years of cash and flexible expenses has historically endured even the worst U.S. starting years on record. A retiree drawing 5% with no flexibility and no buffer can be permanently damaged by just two or three bad early years. The lesson is that your defenses matter more than the raw sequence: buffer, flexibility, and a conservative early rate together let a portfolio ride out prolonged downturns.