Which browser plugin technology to use for cross platform design?

I mean ActiveX is IE only and dead, and so java applets are dead. Flash is still not present in mobile browsers, Mo开发者_高级运维zilla is present only on 1 phone, so XPCOM is also not an option.

How do you write a plugin that need access to file system, and do some other things, like unzip files, upload, download, etc.

Forget about client side plugins. Most of that can be handled on the server side. Then for anything that really does need a client side implementation you can use Javascript.