Getting people to pay for something that has always been free is a tall ask. Most people are barely aware of what a browser is. They just think it’s part of the OS.
There's actually an advantage to the paid business model vs ads in that you don't have to appeal to N million people in order to pay the bills: you only have to appeal to `expenses / subscriptionPrice` people. This means you can cater to those people more aggressively and turn them into fans rather than just users, while also saving time on the features they don't need (reducing `expenses`).
(I'm a happy subscriber to all three above-mentioned services and would immediately sign on for a paid Firefox fork like OP suggests.)
However when it comes to web browsers, there’s been a looong history of failed attempts at selling commercial browsers.
I wouldn’t be at all surprised if the only people who’ve ever made any serious and sustained income from browsers have been Google; and even that’s been indirectly via upselling their other services.
Times change. Subscriptions are normalized, and tech people are increasingly aware of the hazards of "free".