Page still scrolls horizontally even though I set overflow-x to hidden

I am building a website that has a javascript slider which lines all the photos up horizontally and slides them into view, so to ensure that the user can't see them I set the overflow-x: to hidden which works as long as there was nothing that needed to be scrolled vertically. As soon as the content extended below the window and it needed to scroll, I was able to scroll horizontally too. This only happens in Firefox and I am using a macbook pro with a trackpad that allows me to scroll horizontally. A horizontal scroll bar doesn't actually appear. Anyone with any code or a fix would be my hero!