Mozilla files IRS form 990 ("Return of Organization Exempt From Income Tax") annually due to their nonprofit status. This can be found at the bottom of their "State of Mozilla 2018" annual report [1] and contains some details about their financials (as of November, 2019 at least). The linked page itself also contains some information about how Mozilla Corporation and Mozilla Foundation are structured and operate.

[1] https://www.mozilla.org/en-US/foundation/annualreport/2018/