Mozilla Firefox is undergoing a lot of changes these days. Mozilla developers are removing support for NPAPI plugins from Firefox. The change has already reached the Nightly branch of the browser.
For end users, this means that plugins like Silverlight, Java, Unity (a framework for games) and Linux's Gnome Shell plugin will stop working completely. Firefox 52 already has them disabled out of the box, but the user can turn them on using about:config. With Firefox 53, the ability to restore NPAPI plugin support will be removed completely. Firefox 52 is expected to be released on March 7, 2017. It will also be available for ESR branch users. Firefox 53 will be out on April 18, 2017.
Mozilla has made an exception only for Adobe Flash. Lots of web sites still rely on Adobe's Flash Player technology, so they decided to keep it. If these web sites stop working in Firefox, this can cause Firefox users to switch to another browser.
In the future, Mozilla will remove all NPAPI code from the browser, removing all plugins including Flash. Instead, they are going to implement PPAPI plugin support. Flash is also available as a PPAPI plugin so it could possibly still work in Firefox.
Other modern browsers like Chrome and Chromium-based apps have removed NPAPI support already a long time ago.
You can inspect which NPAPI plugins you are using right now by typing the following in Firefox's address bar:
How many NPAPI plugins have you installed? Which ones are you using actively? Tell us in the comments.