You are probably behind a router. Your IP address is in the private address block (192.168.xx etc)
You should find your router and do a hard reset to make sure there are no dodgy settings within that. When you have done that, make sure you enable any security settings within the router such as firewalls etc, and disable UPnP if on there.
As for your PC, remove and reinstall firefox with no extensions and try again. Hopefully this should help
Edit: To hard reset router, locate the small hole usually on the bottom of it that says reset next to it and use a pin or other thin object to press the button in whilst the router is still switched on. The lights may flash or go out and come back on depending on model etc.
The router should then reset with good security settings already enabled (unless it is a wireless router also in which case you should enable WPA security) to do this type into your web browser 192.168.2.1 or whatever the ip address of the router is.