If you create a brand-new web application in ASP.NET 2.0 or later, client-side validation will automatically work in Firefox. However, if you have legacy applications originally written in ASP.NET 1.1 or earlier, client-side validation will not automatically work in Firefox. To enable this feature in legacy applications, simply open your web.config file and locate the xhtmlConformance element and change it to this:

<xhtmlConformance mode="Transitional"/>