Java in Mozilla after ver 52
However, there is a way to continue using NPAPI plugins in Firefox - use Firefox ESR (Extended Support Release) instead.
The latest Firefox 52 ESR continues to allow using NPAPI plugins, and it is supported until May 2018.
Update (thanks to grof for the tip!): another way to enable NPAPI plugins other than Flash, which works with Firefox 52 (not ESR; will probably not work with future Firefox versions) is to type "about:config" (without the quotes) in the address bar, then right click, select New > Boolean, enter "plugin.load_flash_only" (without the quotes) as the new Boolean value and set it to false. Once you're done, restart Firefox.