The negative impact of NPAPI plugins on a browser’s security, speed, and stability, and the complexity of the code base is the actual reason.
Google Chrome 42 release ended its support in April and Microsoft also droppeded Silverlight with the launch of Edge in July this year.
Benjamin Smedberg, Firefox’s quality engineering manager stated via a blog post:
“As browsers and the web have grown, NPAPI has shown its age. Plugins are a source of performance problems, crashes and security incidents for web users. “
“Mozilla intends to remove support for most NPAPI plugins in Firefox by the end of 2016. Firefox began this process several years ago with manual plugin activation, allowing users to activate plugins only when they were necessary.”
“This decision mirrors actions by other modern browsers, such as Google Chrome and Microsoft Edge, which have already removed support for legacy plugins.”
Mozilla kept the state of Flash alive stating since Adobe Flash is still a common part of the Web experience for most users, its support will continue within Firefox as an exception to the general plugin policy.
Also, the collaboration of Mozilla and Adobe will continue for improving Flash experience on Firefox with stability, performance, and security.