Many purveyors of FIRE (Financial Independence, Retire Early) have large incomes. These are people such as doctors or lawyers who make large salaries but live a modest, middle class lifestyle. This allows them to pile up money and reach financial independence in their thirties or forties.
Others reach FIRE through extreme frugality. Reducing their expenses to nothing so that they can save and invest despite a relatively modest income. Even once they retire, they maintain a modest lifestyle so that they don’t need a big income. Therefore they don’t need to save up so much to reach their early retirement goal.