Add the JSObfu mixin to Firefox exploits

##### Verification

- [x] Get a shell from any of the following Firefox exploits, making sure to `set JsObfuscate 1`:

    exploit/multi/browser/firefox_proto_crmfrequest
    exploit/multi/browser/firefox_tostring_console_injection
    exploit/multi/browser/firefox_webidl_injection

- [x] Run the post modules following on the session, making sure to `set JsObfuscate 1`.

        post/firefox/gather/cookies
        post/firefox/gather/passwords
        post/firefox/gather/history
        post/firefox/gather/xss

- [x] specs pass