Debian installs Firefox to an unwritable directory. You can't add extensions to FF w/out write permissions. You could get the installer from http://mozilla.org/ which is how I prefer to install FF.

Code:
wget http://ftp.mozilla.org/pub/mozilla.o...staller.tar.gz
tar xzvf firefox-0.9.1-i686-linux-gtk2+xft-installer.tar.gz
cd firefox-installer/
./firefox-installer
This sets up firefox in ~ (/home/username). You can easily add extensions, themes etc. w/out worrying about root permissions. To run it do:

Code:
~/firefox-installer/firefox-bin
You can create a script to launch it for you:
Code:
touch firefox.sh
echo "/home/username/firefox-installer/firefox-bin" > firefox.sh
chmod +x firefox.sh
sudo mv firefox.sh /usr/bin
Now just create a luancher on your desktop or taskbar to run your new script by executing:
Code:
firefox.sh