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".
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.)