Since JNLP files are simply Java Web Start files that open applications through an Internet browser, any browser that works in Windows 8.1 will open a JNLP file as long as you have Java installed.

Java Network Launching Protocol, or JNLP files, are used for launching Java programs and Web applets over the Internet or company intranet. While some browsers, such as Internet Explorer, launch JNLP ...