> The FIRE movement, it seems, is only for those who have a reliable and predictable monthly income, or for those who can assume somewhat 100% employment till 40. If you're an entrepreneur, this arbitrary 50% saving of income won't work. You're more likely already forced to save for the next 3 months's just for wages and electric bills.

I don't think you're viewing this quite right. There's no reason you have to hit a magical 50%. Having a regular income just makes it easier to predict when you can retire / when you become financially independent.