Install lastest stable Firefox from PPA

wget https://raw.githubusercontent.com/panticz/installit/master/install.firefox.sh -O - | bash -

#!/bin/bash

# add repository if necessary
RELEASE=$(lsb_release -rs | tr -d ".")
if [ ${RELEASE} -le 1104 ]; then
  sudo add-apt-repository -y ppa:mozillateam/firefox-stable
  sudo apt-get update -qq
fi

# install firefox
sudo apt-get install -y firefox

# install language packs
sudo apt-get install -y firefox-locale-de firefox-locale-en

Global Firefox extensions folder
/usr/lib/thunderbird-addons/extensions/

OPTIONAL: install flash
wget http://fpdownload.macromedia.com/get/flashplayer/pdc/11.2.202.233/install_flash_player_11_linux.x86_64.tar.gz -O /tmp/install_flash_player.tar.gz
tar -xzf /tmp/install_flash_player.tar.gz -C /tmp
cp /tmp/libflashplayer.so /usr/lib/firefox-addons/plugins/

OPTIONAL, install german language pack
http://www.gtlib.gatech.edu/pub/mozilla.org/firefox/releases/3.6/linux-i686/xpi/de.xpi

Leo translate engine
http://dict.leo.org/pages.ende/toolbars_de.html

Gmail Notifier
http://www.nuko.org:4649/addons/temp/gmail_notifier-0.6.4.3.privateBuild1-fx.xpi

Links
http://www.daniel-ritter.de/blog/ubuntuzilla-immer-die-aktuelle-firefox-und-thunderbirdversion-unter-ubuntu