mv /usr/bin/firefox /usr/bin/firefox-old
sudo tar xjvf firefox-26.0.tar.bz2 -C /opt
ln -s /opt/firefox/firefox /usr/bin/firefox
The above is how I did it, if you don't do it properly you'll find that you are still using the old version (so verify in About that it is the new version). I've verified the above method works perfectly, it's great if you're using an older version of Linux and need to update the most commonly used programs like Thunderbird and Firefox.
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