It's not just impossible for you to specifically fund Firefox. From what I understand, Mozilla Foundation money does not go to Mozilla Corporation/Firefox at all. You cannot donate to it at all, and your donations only go to those things you don't want to fund.
Your understanding is accurate. Donations to Mozilla cannot legally be used for Firefox development because your donations are tax-deductible due to Mozilla being a charity and Firefox development is done by a for-profit corporation.
