On Chrome my cart page has a big gap on the right, but on FireFox it’s nearly cutting off the text.
Chrome:
Firefox:
there are horizontal scroll bar on the Firefox, are you sure that zoom is set to 100% in both cases?
I didn’t think to check that, but I just did, and yes it’s both are at 100% 
are the cart set to fixed or fluid? also, is it stretched to the section size?
if it is stretched to the section, could you try to add paddings on the left side of section?