hi Sajjad.h, you are using a 64bit version of firefox. the only plugins it supports are flash player and silverlight. so if you'd like to install the unity player you'll have to stick to the regular 32bit firefox release for windows.

as announced already last year mozilla will generally remove support for npapi plugins (like unity web player) starting in 2017. unity is also pushing publishers to switch to web technologies running natively in the browser without the need for any plugins...

https://blogs.unity3d.com/2015/10/08/unity-web-player-roadmap/ https://blog.mozilla.org/futurereleases/2015/10/08/npapi-plugins-in-firefox/

Note Plugins are on system and not installed in Firefox like Extensions and Themes (complete) are. To see where the Plugins Firefox is making use of is located on system type about:plugins in Location (address) bar like you would type www.example.com