hi,
Looks like you have made a terrible hard-coding.
See the screenshot to understand what the problem is:
Attachment Pasted_Image_9_16_13_1_56_AM.png not found
It works fine with Safari and Chrome simply because those browsers don't care about button's target attribute in a the form. It always takes into consideration the <form>'s target attribute. But Firefox does.
Anyway, you need to remove this
target="_blank" from there.
Open this file:
components/com_virtuemart/views/cart/view.html.php
Find that line below in this file and remove the
target="_blank" at the end
$checkout_link_html = '<a class="vm-button-correct" href="javascript:document.checkoutForm.submit();" ><span>' . $text . '</span></a>';
Regards.
Support & Development