Most likely, a web browser extension (such as AdBlock) is interfering with your attempted purchase. We would recommend disabling all web browser extensions for Firecracker, and/or trying another modern web browser (such as Safari or Firefox). 

On Chrome's newest version, there is a vertical stack of three dots in the upper-right; you can hit that and then go to 'More Tools' to manage your web browser's extensions.

Firecracker accepts most major credit and debit cards, including MasterCard, Visa, American Express, Discover, and JCB.