Plugins Vs Extension

Extension and plugins often confused people. And Mostly people believe both are same.
However, there is subtle difference between Extension and Plugin.  Extensions are mostly written in JavaScript and/or XUL and relies on XPCOM APIs to access resources. Whereas, plugins are binary applications and they make use of operating system and can do interaction with operating system or access events from OS.

Securing plugins is more complex than extension.  Extension are individual identity so one extension needs one set of privileges, whereas plugins are more complicated and needs to identify privileges for each application runs plugins rather than a whole plugin.