That's an interesting question! Paying for things is the obvious answer, but us users have been trained to feel entitled to not paying for other people's hard work. So the most interesting one in the space of "how to get people to pay for a web browser" is Replay.io, which is "just" a firefox re-skin (it's infinitely more than that) selling to a niche audience that is known for being cheap. So rather than sell to them directly, sell to their employers (it's a business tool) and get into the enterprise space, which is its own can of worms. It's a ridiculously powerful tool, if only their target market could manage to hold it correctly.