Nice idea, but...
I really don't want the task of making online payments to be made so easy that it short-circuits my decision-making processes - which of course is what the advertisers and sellers want! As a web dev myself, I can see both sides of this.
By way of explanation, let me draw a comparison between buying in a shop and buying online: The difference is that in a shop, the buying decision is made at the shelf the product is stored in. You physically look at the product, decide if you want it or can afford it, and then put it back or put it in your shopping trolley accordingly. By the time you get to the checkout, the decision has pretty much been made, because very few people back out of buying something once they're at the counter. There's a kind of "public shame" associated with backing out of buying stuff once you've arrived at the counter for the cashier to ring up.
Online, however, you can click items into your cart willy-nilly, because come checkout time, there's no "public shame" and backing out of the purchase process at the last moment is easy. I know, because my job involves developing and maintaining e-commerce sites and "checkout abandonment" is a lot more common than in bricks-and-mortar shops. Finding ways of reducing this checkout abandonment is a major design issue for any e-commerce dev team, and it's what's behind this push by Mozilla.
For me, when I buy on line, I add things I like the look of to the cart as a way of "bookmarking" items I might want to buy; and the decision-making process comes when I am confronted with a credit-card entry form and see the total amount I'm about to spend. Do I really need to buy this right now? If not, just back out of the checkout and delete it from the cart. Or just abandon it altogether and keep searching for a better deal.
For this reason, my site designs always include an easy "cart <-> wishlist" transfer button next to each entry, so customers can instantly transfer items they don't want right now into their wishlist, allowing them to come back and buy it later if they wish. It's not a sale, but a wishlist is a whole lot better than an abandoned cart. And they can do that all the way up to the credit card form.
Conversely, when I'm in a bricks-and-mortar shop, I make the buying decision standing in the aisle looking at the product, because I know that once I head to the checkout it's like a psychological point of no return.
So for me, and probably for many others, eliminating the credit card form takes away that stop point. One easy click and bam - I've just spent way more than I meant to. Which, of course, is exactly what the sellers want; to eliminate the decision making process as much as possible to profit from all those impulse purchases. As a site developer, it's my job to make that happen.
And that is why, as a customer, I prefer to have to enter my credit card number for online purchases. It's where you stop and think about the fact that you're about to spend money; an equivalent of the in-shop counter - do I have everything I need, do I need everything in my cart? Take that away, and I suspect a lot more people will come to grief over overblown credit cards than is even happening now.