I stopped using Chrome when they added the “feature” that logging into gmail was the same as logging into the browser.
If there was a little notification to the top right in the Firefox with a link to “pay”, even if it was optional, there is some proportion of users who would. And I would bet that your average Firefox user is an order of magnitude more likely to do this than your average chrome user.
So I wonder why they don’t do this? I’ve had similar frustrations for other products I get for free that I wanted to pay for.