I was just poking around on a Shopify store on my Firefox browser and saw Apple Pay button showed up, first thought it's a front-end bug on the website but tapped on it and the widget slid up and you can legibly make the purchase!
Tried on Chrome too and it worked there too!
Tried a few other website and can confirm it works everywhere Apple Pay is offered. is it a feature? bug?
Wanna try it yourself? you can check this Stripe test page https://stripe.com/docs/stripe-js/elements/payment-request-b...
Haven't installed Beta 4 yet to see if it is still working.