Comment by auxide
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.