You're attacking two strawmen:
1. The average user doesn't have to pay for the browser in a donation model, you just need enough users to feel passionately enough about it to fund it sufficiently to develop it.
2. No one is arguing that Mozilla should replace their revenue from Google overnight with donations. We're just asking that Mozilla give us the option to pay for Firefox already.
Another user (trying to demonstrate to me that donations would never be enough [0]) figured that if we assume a similar rate of donations as Thunderbird gets then Firefox would bring in $70m/year just in donations.
That is a heck of a lot of money. That funds 140 developers even at inflated Bay Area salaries, 280 developers if you're willing to branch out of the Bay and offer closer to $200k/year on average as a base salary (still an insanely high average rate in most of the country and the world). Even if you took a full 50% for general/administrative and overhead, that sum would still pay for 70 bay-area or 140 rest-of-the-world developers.
If Mozilla really does need more developers than that for Firefox specifically, then fine, they can keep accepting money from Google—no one is saying they should only be funded by donations. But that they don't even make it an option is frankly bizarre.
[0] https://news.ycombinator.com/item?id=40901664