It's certainly a feel good idea but the math doesn't math. Even a best case scenario donation drive would never be able to compete with search licensing revenue.
Right now if you want to look at the best case scenario, it's probably wikipedia. Wikipedia is the biggest online donation drive that exists and they get 18% of the revenue that Mozilla gets from search licensing on approximately 4,400% more global traffic. And it's a mature campaign that's been an annual tradition for decades.
I'd rather have the donation option than not have it but it has to be understood primarily as something that's just there to make users feel good. The reason they're structured the way they are is to access the search licensing revenue that gets them income they would never be able to get from just donation drives.
I agree that switching over to an exclusively grassroots sponsorship level would be impossible.
But there may be hybrid options available.
Purely conjecture. Does the Google money come with some kind of funding non-compete? If not, why not open up other funding streams. If it does, that's worrying.
The argument has always been "the org structure doesn't support donations" but the org structure is just a proxy for the intentions of the org.
(edit - to allow users to fund Firefox, allowing us to better sleep at night, and to align our incentives)