Debian by default comes with Iceweasel web browser instead of firefox. Although it is recommended to use Iceweasel you may have a need to install firefox are here is a simple way how to install firefox on Debian wheezy by using Linux mint's debian import repository. First edit your /etc/apt/sources.list file and add the following line:
deb http://packages.linuxmint.com debian import
Update your package list:
# apt-get update
The above command will fetch mint package repository list. As a last step install firefox browser:
# apt-get install firefox
You may want to disable Linux Mint's package repository after Firefox installation by commenting a newly added line in apt sources.list.