Firefox only: left/right arrows move the entire page

This is a bizarre issue happening with my website on Firefox only. Using mouse scroll, I can only move up and down. That's the intended behavior. However, if I use arrows I can move the entire page to the right in what seems like ~40px increments. This creates a block of white space along the right edge of the browser window. I can also move it back to the left until the page is in the correct position (but no further -- ie, I can't create a white block on the left).

I don't want the left and right arrows to move the page like this... it happens to all of my site's pages (even the ones that are absolutely positioned). It only happens in Firefox (49.0.1).

Does anyone have any clues for me? thanks.