[CentOS] Firefox 68 jnlp files

It seems that firefox 68.x, as distributed with CentOS6 updates, no longer allows opening jnlp files with javaws

The "Choose Helper Application" window popping up after selecting "Open with/Other" has a predefined list of applications that cannot be customised.

I'm sure it must be a configuration issue as this works fine with the same browser on other distributions, but what to look for?

# alternatives --display javaws          
javaws - status is auto.
 link currently points to /usr/java/latest/jre/bin/javaws
/usr/java/latest/jre/bin/javaws - priority 200000
Current `best' version is /usr/java/latest/jre/bin/javaws.
# ll -L /etc/alternatives/javaws
-rwxr-xr-x 1 root root 140296 Dec 15  2018 /etc/alternatives/javaws
#