When you must, follow this guide to minimize risk.
# Extract to a custom location tar xjf firefox-52.0.2.tar.bz2 -C ~/firefox-old/ # Run from extracted folder without affecting system Firefox ~/firefox-old/firefox/firefox --profilemanager # Create a new profile called "oldprofile" # Launch with that profile ~/firefox-old/firefox/firefox -P oldprofile -no-remote Newer Firefox versions automatically upgrade old profiles. If you share one profile, your old Firefox will corrupt it for newer versions. firefox old version