A very, very weak sales pitch. I've seen more things start prominently displaying that they're "AI-free" recently, and it has only driven me away as opposed to being more interested because do you just not stand out enough to the point where you have to say that in order for people to care? Or is it because you're stuck-up? I'm not sure anymore.
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 "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.