I have 15.6 stable installed and when opening the Stripe test page I receive an error in Firefox (“Either your browser does not support the Payment Request API, or you do not have a saved payment method.”)
iOS 16 beta apparently provides the framework updates necessary for third-party iOS App Store browsers to use it. So you won’t be able to use it on 15.X unless they backport, which is unlikely.
Stripe's demo doesn't work on desktop Firefox, either.
> Either your browser does not support the Payment Request API, or you do not have a saved payment method. To try out the Payment Request Button live demo, switch to one of the supported browsers below, and make sure you have a saved payment method.
Desktop Firefox doesn’t use the Apple WebKit engine on the backend, so that’s up to the Firefox Desktop team to implement on their own (and tie into the OS native if available, which I suspect depends on you running the latest macOS beta, assuming they’re working on it yet).