This relates to the protocol handler feature implemented by Deploy for JDK9 so javaws is launched directly.

https://bugs.openjdk.java.net/browse/JDK-8055464

Here is the Linux version:

https://bugs.openjdk.java.net/browse/JDK-8129792

Here are the manual steps that need to be documented in the docs since this cannot be programmatically setup by the installer.

Firefox:
1. Open Firefox
2. In location bar type "about:config"
3. Add new boolean
    name: network.protocol-handler.expose.jnlp
    value: false
4. Add new boolean
    name: network.protocol-handler.expose.jnlps
    value: false
5. You will be asked to choose the application to launch once you click on a jnlp:// or jnlps:// protocol. Browse to javaws and clock OK.


Chrome:
To configure xdg-open:
1) Create javaws.desktop in .local/share/applications with following (make sure to change PATH_TO_JRE to path to JRE):
[Desktop Entry]
Encoding=UTF-8
Name=Java(TM) Web Launcher
Exec="PATH_TO_JRE"/bin/javaws %U
Terminal=false
Type=Application
MimeType=x-scheme-handler/jnlp;x-scheme-handler/jnlps

2) Run following commands:
xdg-mime default javaws.desktop x-scheme-handler/jnlp
xdg-mime default javaws.desktop x-scheme-handler/jnlps