The whole Mozilla foundation budget oscillated around $100-120M/y for last few years. Let's assume that half of it was dedicated to Firefox; e.g. $60M/y. It would take 500k users paying $120/y (aka $10/mo) to support their favorite browser. The current audience of Firefox is approx. 170M users; it would take about 0.3 percent of the audience to be paying users; 0.6% if you lower the rate to $5/mo.

This is how any freemium works.

Even more funnily, someone with a good reputation could just start an organization to accept the payments and direct them to Mozilla developers, both Mozilla employees and significant open-source contributors. Eventually the developers might stop needing the paycheck from Mozilla, and thus from Google.

Now that's not some optional donation scheme, there are real tangible benefits to being a paid subscriber, so idk how that could fit into something like Firefox.