works properly with Safari, but not with Chrome and Firefox: problem with the cart

I did all the tests with Safari, as I 'm a Mac Addict and the plugin is done with people "who know".
But, , tonight, a friend of mine, told me that it doesn't work nor with Chrome, nor with Firefox. In fact what don't work are the plugins I used to print the cart: MP Floating Cart and MP Custom Shipping Form. Ok, they are not exactly yours, but I have found them because I 've read all I 've found about market press on this website.
So I need a solution.