Apple are partly to blame, but the trend for 'full screen background' websites doesn't help. If the content obviously continued below the fold you'd scroll without needing a prompt. Websites shouldn't be designed in ways that make you have to learn to check if there's more content hiding under what you're seeing.
I never have the issue on Firefox on my Linux laptop, but I always run into issues with not knowing to scroll on my Mac/Safari, simply because the scrollbar doesn't exist. On Linux, a quick glance to the right and I know that there is extra content that I can scroll to see. That doesn't exist on Safari. It's annoying, even if it is pretty >_>