The money flow between the for-profit that develops Firefox and the non-profit foundation is one-way: From for-profit to non-profit. This is because it is illegal for the non-profit to give money to the for-profit.

Any donations you make go to the non-profit. They are not used and cannot legally be used for Firefox development.

Edit to add: Mozilla chose this legal structure. Mozilla chooses to disallow donations or payments directly to the for-profit. For example, nothing prevents using a shareware model, where Firefox is free but you can choose to pay for it. And Mozilla chooses to avoid mentioning this structure when accepting donations from you.