The software repository is a pretty nifty thing, but sucks when there's something new you really want and it isn't there.
To install it, go to the terminal and type:
1.
sudo
add-apt-repository ppa:mozillateam/firefox-stable
2.
sudo
apt-get update
3.
sudo
apt-get
install
firefox
This should replace your current Firefox with the latest stable version.
[ Modified from Source so it doesn't upgrade EVERYTHING ]