Well...this is not a simple question to answer...as most programming languages (beside Java) are created for a specific reason.
Learning Java on the other hand will give you greater opportunities..as you can program for the web, mobile devices, desktop applications, client/server applications etc.
Having said that though, you're ought to ask yourself what it is that interests you the most in programming languages....and what jobs to you see yourself doing if you are to become a full time programmer.
As for plug-in programming...etc. there are loads of howto at the mozilla foundations website which explains the process of creating plug-ins for the mozilla family of products. here is a nice howto from the mozilla documentation pages.
Here is quite a nice introductory tutorial on how to create a plugin for Mozilla Firefox