I’m facing the same problem using Windows 7. When I try to buy Gem’s the Paypal Webside doesn’t open in my browser. I already installed a new browser (Chrome) and set it to my default browser. That didn’t worked. So I deinstalled Firefox and installed it again, setting it to my default browser. Nothing happens (even if the browser is already open or closed).
Don’t know. I will try to deinstall GW2 and install it again … maybe this helps.