Note that if you plan to downgrade Firefox because your Selenium tests broke after a Firefox upgrade, there is a better way that doesn't involve downgrading. Mozilla has stated that they will no longer provide security patches for any but the most recent versions of Firefox. So running an old Firefox should not be a long-term solution for anything.
If you still want to downgrade your Firefox for other reasons, here is how I downgraded my Firefox 6 to Firefox 5. Doing this will probably fuck up your system and my Firefox text rendering looks like shit now. Anyway, here it goes:
CTRL+Shift+O
) to make a backup of your bookmarks. I didn't have to use it, but better safe than sorry.tar jxpvf firefox-5.0.1.tar.bz2
sudo cp -R firefox /usr/lib/firefox-0.5.1
sudo unlink /usr/bin/firefox
sudo ln -s /usr/lib/firefox-0.5.1 /usr/bin/firefox
You probably want to disable auto-update in your freshly downgraded Firefox.