@CliftonR Well, Tusky doesn't delete the prefs on all downgrades. If it does it, it only does it when the *prefs file format* changes, which doesn't happen on all version upgrades. But it could be that Firefox's version file really does change on every single version update.
@mcc
Yeah, I wasn't thinking about what you're doing with Tusky, though it's obvious why it's in the forefront of your thoughts.
Having a version number for your pref format and checking it's <= the supported version is an eminently sensible starting point.
I implemented something similar to that at $DAYJOB for configuration/pref files years ago.
I'm mostly just musing about how Firefox could do better if they wanted to.