How to Troubleshoot media queries on different browsers?

I have a website (http://ift.tt/1xkHI9N) that works just fine most of the time...but sometimes the browser (Firefox and Chrome) will determine the wrong screen size and show the site wrong. The same computer will show it differently on two different browsers: one browser will determine that the screen is 960-1240 and the other will correctly see that it's over 1240. I can't figure this out. Anybody have some good tools for troubleshooting this issue, or better yet, know what the problem might be?