(First, you are running an old version of Firefox. Please update to Firefox 55 ASAP, 53 has known security issues). What I imagine you are seeing is an unsecure website error. This usually happens when a website isn't using SSL or hasn't properly configured it. This means that people on the internet have the ability to get access to your connection to that website. There are many more possible points where your connection can be snooped on than just your computer and router. Keep in mind, the best way to fully secure your connection is to Keep your Web browser, your OS, and your router fully updated. It's too late for you to get the free upgrade to Windows 10 unfortunately, but you should still install every update you can.