Fix condition

Author: jvazquez-r7

modules/exploits/windows/browser/mozilla_firefox_onreadystatechange.rb

@@ -92,7 +92,7 @@ def on_request_uri(cli, request)
 			return
 		end
 
-		if agent !~ /Firefox\/17/ or agent !~ /Firefox\/21/
+		if agent !~ /Firefox\/17/ and agent !~ /Firefox\/21/
 			print_error("Browser not supported, sending 404: #{agent}")
 			send_not_found(cli)
 			return