I gave a quick look over this.
When trying to compil icedtea-web it requires a "mozilla-plugin" package.
From what I understand, after changing names many times this "package" or "headers" are now in NPAPI. Problem is: Chronium and Chrome are not using this anymore and Firefox plans to stop NPAPI soon.
https://blog.mozilla.org/futurereleases/2015/10/08/npapi-plugins-in-firefox/
Anyway didn't try with NPAPI headers yet but this could be a future problem, I'm not sure icedtea-web plan to evolve with the new API chrome et Firefox are using.

IcedTea also works stand-alone outside the browser to launch .jlnp files. I use it this way to access Cisco ASDM for administering ASA Firewalls as well as the remote console feature on SuperMicro servers.

In T962#41363, @ben-kuhn wrote:

IcedTea also works stand-alone outside the browser to launch .jlnp files. I use it this way to access Cisco ASDM for administering ASA Firewalls as well as the remote console feature on SuperMicro servers.

yes that's true that's why I use it for jlnp files for my uni they are using blackboard