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.
On the other hand, the Google money accounted for around 85% of their income over the years, so if they hadn't been spending it they would have been operating on around 20% of the income for many years while the endowment grew, and likely would not have been able to keep up with competing browsers.
Also, for as much crap as she gets, Mitchell Baker invested over 20% of the Google money Mozilla received during her tenure, far more than was invested by prior CEOs. And before anyone brings it up, all that "woke activist" spending comes from donations, not Google money, which the IRS prohibits them from spending on browser development.