Amberly Grant, 36, considers herself 'coast FIRE.' Courtesy of Amberly GrantFinancially savvy millennials are maxing out HSAs to supplement retirement savings.HSAs require enrollment in a high-deductible health plan, suitable for healthy individuals.In 2024, individuals can contribute up to $4,150 in an HSA.When Amberly Grant started making enough money to invest, she opened and maxed out three different accounts: a 401(k), a Roth IRA, and an HSA.The first two are common retirement-specific accounts.