One-Page Checkout [Support Thread]

[mprough]

no need for the inline style "display:none"

this is also set in checkout_one.css
#checkoutOneLoading { position: relative; height: 95vh; top: 50%; left: 50%; display: none; }

Removed the display none styles, in what you mentioned as well as tpl_checkout_one_confirmation_default.php and still was not displaying in Firefox, but I changed

#checkoutOneLoading { position: relative; height: 95vh; top: 50%; left: 50%; display: none; }

to

#checkoutOneLoading, #checkoutOneConfirmationLoading { margin: 0 auto;}

Still that gif will not display in FireFox