One approach to coping with quantum is to run 2 (or more) Firefox profiles, one for the old, or ESR, or pre-quantum executable; and the other for Quantum: /usr

One approach to coping with quantum is to run 2 (or more) Firefox profiles, one for the old, or ESR, or pre-quantum executable; and the other for Quantum: /usr/bin/firefox53 -P esr /usr/bin/firefox -P quantum and if you want access to the same saved bookmarks, just use Firefox Sync to match them up. It won't help with Pentadactyl, but folks with other use cases may find it useful.