> But as all the calls are Synchronous calls, firefox throws a message stating "Unresponsive plugin".
> Can anybody suggest any solution/alternative, for above scenarios.
The only general (and recommended) solution is to make the calls asynchronous.
Every plugin call that needs to do some processing or has to wait on something should be asynchronous
and notify the JS when it’s done.
> As chrome has already restricted npapi plugins, going ahead, will firefox
> also follow suit. In that case, what is other alternative ?
Firefox will make plugins click-to-play by default soon (currently planned for Firefox 31)  .
What do you want to do?
The alternatives really depends on that - there might already be a suitable WebAPI or one is being