ubuntu comes with firefox (stable) built-in. I wanted to try Aurora, so I added ppa:ubuntu-mozilla-daily/firefox-aurora to the source list and upgraded this way :

sudo add-apt-repository ppa:ubuntu-mozilla-daily/firefox-aurora

sudo apt-get update

sudo apt-get upgrade

now let us suppose that I want to go back to the stable version of firefox, what would be the safest way to do so ?

thank you.

Edit :

As pointed out by @vasa1, I am not talking specifically about firefox, but about the concept of going back to a lesser version in general.
I have google-chrome installed so I guess, Ubuntu will one end up browserless.

Problem solved using @IgnitE's solution.