Mozilla is set up as a non-profit with a for profit company as a subsidiary. The corporate Mozilla handles working on Firefox, mostly using money from Google for setting it as the default search engine. Because of that separation I don't think they can easily mix those two piles of money together.

There's this section from their FAQ:

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.

as i understand it, the money goes to the foundation, and it's the corporation that develops the browser. so it's probably not strictly forbidden, but it does imply that the money is not for browser development.