All you need to do is add a PPA and run upgrade and your Ubuntu will be running the latest and the greatest version of Firefox. Open the terminal and run this command:
sudo add-apt-repository ppa:mozillateam/firefox-next

This command will add Firefox's PPA to your repository.

Now update the repository by running
sudo apt-get update

Once repos are updated run this command
sudo apt-get upgrade

It will upgrade all applications to their latest versions. If you have Firefox running close all the windows and then open again. You will be running the latest Firefox. When the next version of Firefox is out all you will need to do is run to update and upgrade command.