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.

I need to run a jnlp file for work and it seems as of Mojave, it is now blocked by Java Security. Apparently in the old days one could just use the applet in System Preferences to lower the security ...