Here is a function to return you the width the browser is using for media queries. For some reason, it is a bit different in each browser.

gistfile1.js

function getWindowMediaWidth() {

    if(!window.matchMedia){
        // it should be this, so return it when we can't
        // figure it out. Of course, it does not do a lot
        // of good if the browser does not support media
        // queries.
        return document.body.clientWidth;
    }

    // it should be one of these values starting with the
    // first one that makes the most sense
    var values = [
        document.body.clientWidth, // Chrome returns this
        window.innerWidth, // Firefox returns this because
        window.outerWidth, // this is the same as above on browsers with no window borders
        screen.width // last ditch effort here
    ];

    for(var x = 0; x<values.length; x++){
        if (window.matchMedia("(min-width: " + values[x] + "px) and (max-width: " + values[x] + "px)").matches) {
            return values[x];
        }
    }
}