At what point could FireFox had just invested the money from Google into the SP500 and then just ran the company off of passive income?
Like for 150M$ I bet you could fund browser development for at least a decade and that was just 1 year of income. (of course also burn the entire $150M).
Not sure that's a realistic assessment of the cost of developing a browser. Mozilla gives Software Development exp names as by far the single largest expense at 260M$ in 2023. According to DuckAI 700 out of 750 Mozilla Co employees work on Firefox.
I am sympathetic to the idea that a global remote team, that doesn't pay Silicon Valley salaries could get this done cheaper, and thus would be a better candidate for such an Invest and live on interests approach but 15M$ budget seems infeasible.
Reading directly from the 2023 financial report: Revenues were 653M, Software Dev was 260M and change in net assets was 142M, so 402/653 is spent on the core activities you favour (and that is ignoring that you do need a legal and HR department, and some management, and some marketing if you don't want Firefox market share to fall further).