Welcome to the world of browser differences.
As soon as you start customizing client side resources, you must also prepare yourself for having to dig, debug and test the different browsers for different reaction to your changes, and this case is no different.
There are various tools that you should learn how to use, like FireBug for instance, and for IE, you can use script debugging directly inside Visual Studio. There are also quite useful websites with references and difference reports between browser and the one I most often use is www.dottoro.com, or more precisely, their reference at help.dottoro.com. This is a great reference site. Also, W3Schools is a good reference site.
Hope this helps,