I've been more or less on the FIRE train for three years now. It's very easy when you have a software engineer's salary in Seattle. In my best year I only spent $21,000, but was fairly unhappy - saving money is a lot of work! All chores take longer and require more mental effort to figure out the least-expensive way of accomplishing your goal.
At this point I've actually found areas of technical interest I can expect to get paid for in the future, so I'm not as laser-focused on the goal. If you manage to bring in even a bit of cash in retirement, the safe withdrawal rate calculus tilts considerably in your favor.
To head off people saying "but you won't really be retired!" you should know the in-group has a term for you: the retirement police. The real goal is freedom, and not the type where you're lying to yourself that this is what you want to be doing.