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.

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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:

• Use the bookmark manager (CTRL+Shift+O) to make a backup of your bookmarks. I didn't have to use it, but better safe than sorry.
• Download a release for your system from ftp://ftp.mozilla.org/pub/firefox/releases/5.0.1/
• Unpack: tar jxpvf firefox-5.0.1.tar.bz2
• Copy the files into your system: 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.