The Wikipedia page [1] links to an annual report from 2022 [2] where it is stated (page 6) that the expenses for 2022 for "Software development" were about 220 Million Dollars [X], though I guess that the entry includes software other than just Firefox and there is no breakdown.

[1] https://en.wikipedia.org/wiki/Mozilla_Corporation

[2] https://assets.mozilla.net/annualreport/2022/mozilla-fdn-202...

[X] Plus 58 Million Dollars for "Branding and marketing" and 108 Million Dollars for "General and administrative" ... ouch !