Yes, I added paypal to BP&P. (If I want to add " https://www.paypal.com" it disappears, after clicking on OK.
I use Firefox v62, there HTTP/2 is enabled by default. But you can check (https://tools.keycdn.com/http2-test), that paypal don't use HTTP/2.
I don't think that it's a issue of encryption:
The upper window is from an other bank (with better encrytion) which works and the lower from paypal.