If you run Debian stable, you'll have the default Debian browser installed. If you want a browser with more capabilities and plenty of add-ons (think ad-block and flash-block), then you'll be looking at Firefox.
Sometime ago there was a dispute between Mozilla and Debian over trade mark infringement, which resulted in Debian removing Firefox from their servers and replacing it with Iceweasel.
Iceweasel tends to be several releases behind Mozilla's current Firefox offering. So if you want the latest and greatest iteration of Firefox, you have to download and install it yourself.
Here's a quick run through how to do it.
Download Firefox 14
Go to http://www.mozilla.org/en-US/firefox/new/
As root (superuser) do the following.
Unzip the downloaded tar zipped file.
tar xjf firefox-14.0.tar.bz2
Move the newly created Firefox directory to /opt
mv firefox /opt/firefox-14
Create a symbolic link to launch Firefox
ln -s /opt/firefox14/firefox /usr/bin/firefox
Enjoy fast surfing with thousands of add-ons.