You have downloaded the latest version of firefox from here but you dont know how to use the tar.bz2 file

First move to the Downloads directory and unzip the downloaded file

cd ~/Downloads
tar xjf firefox-xx.x.x.tar.bz2


A new firefox directory will be created containing the extracted files. Move the firectory to /opt/firefox-new

sudo mv firefox/ /opt/firefox-new


You can keep the current version of firefox by changing the folders name

sudo mv /usr/bin/firefox /usr/bin/firefox-old


Create a link to the new version's folder

sudo ln -s /opt/firefox-new/firefox /usr/bin/firefox


Close any windows of firefox if you have any to see the changes

Now you can run the new version of firefox from the terminal