The ping is successful in Windows or Ubuntu?
If ping is successful, then you are connected to the internet. Which means the problem is probably either a fouled up install of Firefox or you need to adjust settings in your network monitor or modem.
To check the modems settings, it should be at http://184.108.40.206
The easiest way to check the network settings is to right click on the task bar >add launcher. a list with icons will come up. You're looking for the network monitor (it looks like 2 small wide screen monitors (usually brown) much like the windows one, but more charming). ad that to the task bar then double click it. It should show a list of possible connections. Click the one which is appropriate and check it's settings.
Have you installed a firewall? If so, what settings is it at? Some firewalls install at default high security level, which could prevent a lot of things from happening in your browser but come up with information that says you are connected to the internet.
Also, are you connected to your modem by ethernet card or USB or some other device?