Open a terminal and type:

sudo add-apt-repository ppa:mozillateam/firefox-stable
Your system will now fetch the PPA’s key. This enables your Ubuntu system to verify that the packages in the PPA have not been interfered with since they were built.

Now, as a one-off, you should tell your system to pull down the latest list of software from each archive it knows about, including the PPA you just added:

sudo apt-get update

Finally time to get install the latest

sudo apt-get install firefox