First you can install it using apt-get, You should update your source list.
sudo apt-get update
sudo apt-get dist-upgrade
but if you want to install it manually, here what you should do;
Download then extract as you did already
move the extracted folder to /opt
sudo mv firefox* /opt/firefox38
If you already had a previous Firefox version installed in the /opt directory, remove it with the following command:
sudo mv /opt/firefox /opt/old_firefox
Set up symbolic links
sudo mv /usr/bin/firefox /usr/bin/firefox-old
sudo ln -s /opt/firefox38/firefox /usr/bin/firefox
if everything goes well then you can delete firefox-old in /usr/bin and old_firefox dolder in /opt