How many developers do you think a browser engine needs? How much do they cost?

But also, the answer to your question is at https://assets.mozilla.net/annualreport/2019/mozilla-fdn-201... and https://assets.mozilla.net/annualreport/2019/mozilla-2019-fo... (the 2020 numbers are not up yet because the audit starts when the tax return is filed and lasts for several months, so isn't completed yet).

Just for comparison, last I checked Chrome's marketing budget was estimated to be in the ballpark of Mozilla's total budget.

Disclaimer: used to work for Mozilla.