Check the Request.ServerVariables("HTTP_USER_AGENT") For IE, the string will contain 'Mozilla' For Firefox, the string will contain 'Gecko' or 'Firefox'
If its a non-server side issue, IE has things called conditional comments... which allow you to do X if IE 6 or IE55 or Less Than IE5 or whatever... Essentially it's comments to the rest of the world, but IE sees them as a psuedo programming block so you can define elements in IE to be Not Commented if certain criteria are met
Something like [asp.net] If Request.Browser.Browser.ToLower.Contains("firefox") Then [javascript] if (navigator.userAgent.indexOf("Firefox")!=-1)