If you go to mozilla official site to download a new one, you might feel pain in the a** to install it then for its a full-package source, not a executable installer.

But with terminal, you can do it like easily. Here are the steps :

To update Firefox along with other Update , type and run these commmands on your Terminal :
sudo apt-get update
sudo apt-get upgrade

Then just type this last command
sudo apt-get install firefox

once it finishs and if you're running firefox, it will alert you to restart your browser as your firefox is upgraded

to make sure the firefox is upgraded, goto  Help - About Firefox