In general, if your spending before retirement is less than your earnings (by definition, this should be everyone planning on becoming Financially Free), a tax deductible investment account will benefit you in the long run.
Lets assume you're going to maintain your lifestyle, same level of spending. You're also heading for FF, so you're saving a lot. Imagine you're making $100k gross, you're being taxed some percentage, you're spending $50k and saving the remainder.
Tax brackets - http://www.bankrate.com/finance/taxes/tax-brackets.aspx - We will ignore tax complexity of deductions / etc.. in the end the lesson is the same.
As you can see from the above link, assuming you're married and making $100k total, you have money being taxed in the 10, 15, and 25% tax brackets. ~25k of which is in the 25% tax bracket.
If you save $10k in your 401k, you can deduct that from your taxable income. You now have 15k in the 25% tax bracket. On that $10k, you didn't have to pay your 25% tax rate. Or, you saved ~$2,500 in taxes.
Now zoom forward 15 years, and you're retired (ignoring inflation to keep things simple). Assuming you maintain the same lifestyle, you need to withdraw $50k per year from your accounts still.
If you withdrew $50k from your 401k, your income is now $50k (as 401k withdrawals are counted as income). You have money being taxed in the 10%, and 15% tax bracket. You're no longer being taxed in the 25% bracket at all. Hey look at that! That money you previously were being taxed 25% on is now being taxed at 15%. You'll never need to pay that extra tax.
While this could be a 50 page document, in the end it's relatively simple. If you're saving money, you are most likely paying into higher tax brackets than you will in retirement. So if you can pay taxes later, that's better for you