A few days ago JWZ had a great take on where Mozilla is today: https://www.jwz.org/blog/2024/06/mozillas-original-sin/
It was spending the fucking money.
Foundations like some cancer groups and the arts have an endowment. Each year they build up their war chest by seeking new funding, but a lot of the money they spend each year is the interest payments on their giant piles of cash. Mozilla could have run in perpetuity on the money Google gave them, but instead they decided to branch out into boondoggles and dipping their hands into the cookie jar.