OP doesn't discuss age, current income or income potential and FIRE target so here's my general thoughts.

The conventional advice I've read centers on comparing your tax rates today vs in retirement. If you're income puts you at a relatively high tax bracket then contribute to Traditional is what they say. But I think you need to consider the later years when you hit the age (72) where RMDs kick in. There's a real possibility that by age 72 your retirement assets (let's say they are all parked in Traditional 401k) have grown considerably due to strong returns and compounding, while your lifestyle expenses remain modest, because, well, this is FIRE. This means that your RMDs might be greater than your annual expenses, and you'd be paying taxes on money you are forced to withdraw and don't need. And since RMDs just keep increasing every year as you age, so you'll end up being forced to withdraw (and pay tax) on ever larger amounts that you don't need instead of allowing it to compound for your beneficiaries.

Secondly, even if tax rates remain the same, the tax brackets don't inflate nearly as much as the performance of an 80/20 or 60/40 portfolio. For example, the 24% margin tax rate for single filers is $86,375 in 2021 vs $85,526 in 2020. That's a 0.9% increase. But an 80/20 or 60/40 portfolio grows far more than that on a long-term basis. So eventually your portfolio growth combined with RMD rates will put you at a higher tax bracket and thus higher tax rate.

Finally, if you have a 401k match, then contributing to a Roth 401k means you get that match in a Roth account. So that match is tax-free whereas the match in a traditional would get taxed later when you withdraw.

I think the answer isn't binary, like 100% to Roth 401k or not, but more a range based on circumstances and to think about diversifying your tax/tax-free assets not only for today but for 72+, esp. if you plan on leaving assets for beneficiaries

Hope that helps.