Mozilla has removed the support for all NPAPI plugins except Flash in Firefox 52. If you’ve found the plugins related to Java, Silverlight and other, missing or not showed in the Plugins section of Firefox Add-ons Manager, Mozilla wants to inform [Bug 1311420 – Consider notifications to users about plugin deprecation] you about the same with a notification on top of the about:plugins page that these plugins are gone and no longer supported by the Firefox browser.
Currently, Firefox 53 (Nightly) is showing this message: ‘Missing Something? Some plugins are no longer supported by Firefox. Learn More‘. The Learn more link takes you to SUMO page that offers some explanation, which is not yet prepared.