[ndriscoll]

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.

[freeAgent]

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.