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. - Source
Tbf, Mozilla donations don’t go to Firefox.
It always struck me as odd that there is no way to give to firefox given that anybody can give to thunderbird directly.