MediaPlguin-v-3.3.37.307204 will not install on 50.0.2
The Amcrest IPM-721 needs this plugin to produce video. I've noticed that it will install on some versions of Firefox and the npmedia3.0.0.3.dll is listed along with MMX 3.0.0.3 if this information helps troubleshoot the issue.
Opaite Mbohovái (2)
Windows 64-bit versions of Firefox only allow for Flash and Silverlight plugins; no other plugins are whitelisted and aren't even recognized.
Stick with the 32-bit version of Firefox if you need any plugins beyond Flash or Silverlight.
Also unlike Extensions and Themes (complete), the Plugins are on system and not installed in Firefox.
Type about:plugins to see where the Flash Player and Silverlight Plugins the Win64 Firefox is making use of are located on your system.