Use native code apis in firefox extension to encrypt passwords

I have an extension which communicates with a native application in windows. Extension(password manager) which takes username and password and sends it to native application to store it in the database. While sending credentials from extension to native application, i want to encrypt them. For chrome extension, NACL (native client) is being used to encrypt and decrypt the credentials before sending to native application. I can implement encryption in javascript, but it can be hacked easily since secret key will be visible in browser, but if i use native code apis for encryption, then keys are not open in javascript. Please let me know some way to embed native code( like Google NACL) in my extension javascript code.