YSK that donations to Mozilla don't go to FF developement (which is done by Mozilla Corp.), but are used by Mozilla Foundation for... other things:

https://donate.mozilla.org/en-US/faq/

How will my donation be used? At Mozilla, our mission is to keep the Internet healthy, open, and accessible for all. To learn how your donation is put to use, click here.

You can find more details about Mozilla’s expenditures and governance here.

Don’t Mozilla products, like Firefox, earn income? Firefox is maintained by the Mozilla Corporation, a wholly-owned subsidiary of the Mozilla Foundation. While Firefox does produce revenue — chiefly through search partnerships — this earned income is largely reinvested back into the Corporation. The Mozilla Foundation’s education and advocacy efforts, which span several continents and reach millions of people, are supported by philanthropic donations.

It is funny to me how the first link doesn't actually tell you how you money is used, but just points to Mozilla Foundation home page.

In short, if you donate expecting that your money helps Firefox, you're doing it wrong.