Comment 12 for bug 272896

the problem is being addressed upstream (see please http://bugzilla.kernel.org/show_bug.cgi?id=11721)
i use a Zyxel 642R, but maybe a few of outdated modem can cause this problem.
anyway, just today a workaround has been found (at least for me, please let us know if it works for you as well) try launching with sudo or as root the following command:
echo 0 > /proc/sys/net/ipv4/tcp_sack
and afterwards give a try to firefox to see if you can navigate internet.
should it work for you, this workaround will last until you shutdown or reboot.
in order to avoid to run that command anytime you start your system, you can write a file (give it whatever name you like) containing just this two lines:
#! /bin/bash
sysctl -w net.ipv4.tcp_sack="0"
and put it in /etc/network/if-pre-up.d (obviously you should cp or mv that file to that directory using either sudo or becoming root)

let me know
aldo