They really fucked up by spending that money.

Foundations often have trusts. Many of the donations go into the trust and most of your operating expenses (perhaps not including fund raising) come out of the interest payments, not the principle.

I was so disappointed when I discovered Mozilla was spending it as fast as it comes in. There will never be a windfall like the Google donation again.

They could still work out a way to renew the Mozilla-Google contract, if they are extremely desperate.

I would be against it and prefer them to focus on their own products and strike alternative funding deals with multiple companies like what letsencrypt does and NOT solely rely on one source like Google.