I use Firefox as well but it shouldn't make that much difference.

Go to www.speedtest.net and see what figures you get.

If you haven't already I would then reboot your modem (turn it off and leave it 5 mins or so and then back on (although 30 seconds is enough for mine) and try again. You may see an improvement as mine sometimes looses it's speed and need a reset.

You may also try reinstalling the drivers for the modem.

Otherwise you may have to get BT in.