To install firefox offline installation you down on here: www.mozilla.com/firefox/channels/
1. Unzip the file tar.bz2 out by moving to the directory you to it (My Download)
cd ~/Downloads/
tar xjf firefox-21.0.tar.bz2
2. Move it to / opt which contains the program as: LibreOffice, Google Chrome, Adobe reader, … all contained in this
Before moving you should delete the earlier version you have installed, if any:
sudo rm -r /opt/firefox
Next is moving: (My version is firefox 21)
sudo mv firefox /opt/firefox21
3. Establishing links
Backup old launcher:
sudo mv /usr/bin/firefox /usr/bin/firefox-old
Create a link to the new version
sudo ln -s /opt/firefox21/firefox /usr/bin/firefox

Lastly, run firefox from terminal.
But after doing the above steps you will not be able to run it (like is not installed) otherwise use or access terminal firefox21 and open firefox directory out. To do this you need Create a custom launcher on Ubuntu.