can npapi do everything that ActiveX can?

I wanted to write a B/S project that client side uses ActiveX or Gecko to Communication with server via network socket. And any browser(ie/firefox/chrome/opera/safari) can use i开发者_JS百科t.

If you want to support every major browser than you need to write both the ActiveX part (for IE) and the NPAPI part (for all other browsers). You can use support both technologies with one DLL though.

An easier way than implementing both yourself would be to use FireBreath, which does that for you and provides an abstraction layer over those APIs.

As you write native code with both technologies, sockets etc. can be used with both. The biggest limitations are with the UI side and permissions due to running code in a host application.

Note that XPCOM as a plugin technology is obsolete.