The "paid Firefox" the author wants already exists, as LibreWolf ships almost the exact same code minus the telemetry, ads, and Google defaults for free. If people wanted that, they'd already be using it. The real problem isn't the business model, it's Mozilla's leadership, which has been compromised to hell and back at this point. No pricing experiment like this will fix the exodus.
Yes, forks do indeed piggyback off of their code; that's the point of free and open source software anyway. And Mozilla, in its current state and current leadership, is not sustainable and still won't be with people paying for Firefox. Its marketshare is dwindling, and people are moving to forks such as Zen or to other browsers like Vivaldi. Adding a paid version will just make that trend go faster. And you don't even need to make a fork, because user.js tweaks such as Arkenfox or Betterfox exist anyway.
The point is that people want to fund the development of the actual browser engine which is more important than the customization scripts that those forks maintain. The engine is what people are worried about.
Aren't they just piggybacking on Mozilla's work though? The point is to make the work that Mozilla is doing sustainable, not to pay someone else to ship a slightly modified version of it.