Additionally, what you're trying to achieve is doomed to fail: even if you'd get Firefox to work that way (which is entirely possible), you'd still face the problem you're targetting specific browsers and resolutions (I use exotic 1360 x 1024). Generally, what you want to do is to make CSS that works equally well on all browsers; although due to the many bugs of IE you probably eventually run into a situatation where Firefox and Opera work equally, but IE doesn't get it at all. Luckily conditional comments technique helps here a fair bit.

You're controlling the style entirely on JavaScript. This is a big no. Make it work without JavaScript, only add JavaScript to give more features. Don't rely everything on it. Every browser supports CSS and CSS is always in use, JavaScript is not.