Install non-free wifi driver on my Ubuntu (Dell N4010)

Motivation

My Dell N4010's wifi driver seems not to work well on Ubuntu.

1. Run command

sudo vim /etc/apt/sources.list

2. Add following lines:

deb http://http.us.debian.org/debian stable main contrib non-free
deb http://ftp.debian.org/debian/ squeeze main contrib non-free
deb-src http://ftp.debian.org/debian/ squeeze main contrib non-free

3. Run commands

sudo apt-get update
sudo apt-get install firmware-linux-nonfree

4. Troubleshooting

If you see some error like:

...
dpkg: error processing archive /var/cache/apt/archives/firmware-linux-nonfree_0.43_all.deb (--unpack):
 trying to overwrite '/lib/firmware/matrox/g400_warp.fw', which is also in package linux-firmware 1.157
dpkg-deb: error: subprocess paste was killed by signal (Broken pipe)
Errors were encountered while processing:
 /var/cache/apt/archives/firmware-linux-nonfree_0.43_all.deb
...

Pay attention to the deb file's location, issuing following command:

sudo dpkg -i --force-overwrite /var/cache/apt/archives/firmware-linux-nonfree_0.43_all.deb

5. If still not working

Follow instructions at http://ubuntuforums.org/showthread.php?t=2220377

sudo rmmod iwldvm #otherwise iwlwifi refuses to stop
sudo rmmod iwlwifi
sudo modprobe iwlwifi 11n_disable=1
sudo modprobe iwldvm

ALWAYS use "modprobe -r" instead of "rmmod". It is a smarter and safer way to remove modules. In the background, it executes rmmod, just smartly ('rmmod' or 'insmod' don't care about dependencies, while 'modprobe' and 'modprobe -r' do).