Fix require error for firefox payload

Oops. I missed a require in `post/firefox/gather/xss`. I moved the explicit requires in the other FF payloads into `msf/core/payload.rb` and added the correct require to the post module.

Verification:
- [x] can start msfconsole on a linux machine without the following error:

        joe@ubuntu:~/metasploit-framework$ 
        joe@ubuntu:~/metasploit-framework$ ./msfconsole
        [-] WARNING! The following modules could not be loaded!
        [-] 	/home/joe/metasploit-framework/modules/post/firefox/gather/xss.rb: NameError uninitialized constant Msf::Payload::Firefox