My first thought is that you need to show us some code, but realising you probably need to narrow things down my second thought is that you need to deal with ASP.NET's browser downscaling.
If you aren't aware of it, ASP.NET has a frustrating feature of considering IE better (really no other way to put it) than Firefox and other browsers. See here and here for some details. Check the rendered source in both browsers (and others!).
Not saying this is the issue, just that it could be.