On September 21, 2015, the underlying browser engine which comes from a third party was updated and NPAPI plugin support was turned off. The Slimjet developers did not release that update until they found a way to enable this to be turned back on.
You can google NPAPI and learn a lot more about NPAPI than I'm going to type here.
For those of you who wear aluminum foil hats, the voices lie, don't believe them!
Mostly my replies are about Windows OS. If not I try to remember to specify Linux.