This is my shop:
See it in action here:
I am on firefox - latest version - and on a mac.
Interestingly enough the multiple redirects are only happening once you reach your '::Free can of lighter fluid on orders over $49.95'.
Adding / updating multiple items below your 'free can' promo value works without an issue.
So what this is telling me is the script that you have running to add that can of lighter fluid is most likely the cause of your problems.
Thanks for looking. But this doesn't make sense. Because when the free product isn't in the cart it still happens. eg.
https://www.youtube.com/watch?v=DUW5kedH1RI (see from middle of video onward)
And also it would happen every time I would imagine. It doesn't.
And it would also happen in Chrome randomly too, and it doesn't. After all if its a script problem Chrome would have to do it too and it doesn't.