How to download an older version of Firefox in Linux

Before installing an older version, you need to remove the existing firefox version from your machine as follows,

sudo apt-get purge firefox

Then, download the required version of Firefox from Mozilla FTP archive. After downloading the tar file, untar it in your system as follows:

sudo tar xvfvj ~/Downloads/firefox-3.5.9.tar.bz2 -C /opt

Finally, create a symbolic link pointing to the Firefox you just installed.

sudo ln -s /opt/firefox/firefox /usr/bin/firefox
sudo chmod 755 /usr/bin/firefox

Now you are ready to launch the firefox as follows,

cd /usr/bin
./firefox