It sounds like jacquesm's hit that point, unfortunately at a time critical spot. Probably won't happen twice though. :)
Thinking about this a bit more, it almost sounds like the Ubuntu supplied Firefox (in the base system) was updated to v57, and likely overwrote the /usr/bin/firefox link.
If that's the case, then the manually downloaded v52 ESR is probably still in /myhome/ff/firefox/firefox.
jacquesm, when you have time to check... see if you can launch the version in /myhome/ff/firefox/firefox directly (instead of using the /usr/bin/firefox link). It'd be interesting to see which version is in there. :)
- install Ubuntu - remove OS supplied Firefox package - install Firefox ESR - have a 'surprise upgrade' - re-install Firefox ESR - have another 'surprise upgrade' - install Firefox ESR in my homedir - symlink it from /usr/bin
So now if it pulls any other tricks I can restore the symlink and call it a day, but for now it looks as if that last move did the trick because since I did that (and dropped all FF domains in through /etc/hosts) it has not done any more upgrades, though there is a chance that it attempted to do that.
The one saving grace in the whole story is that at least the plug-ins that got forcibly removed/disabled had their data survive the whole ordeal.