NOTE: I promised a User called M that I would find out how to get Firefox latest version running. However, this post focuses on installing Firefox 45. I will write a tutorial of how to install Firefox 48 or 49 soon!
Open up the terminal and type:
sudo apt-get update
sudo apt-get install firefox-esr
Launch Firefox 45 using Menu/Internet/Firefox ESR.
Epic Chas Gamer 😎