Between the several HN threads on this, this is just what I was missing. Thanks for clarifying it is the defaults which change. That is completely acceptable. It's like any other feature or aspect of FF that a developer can change.
It gives the developers a great power—they can change applications behaviour outside typical venues for this. Most users expect an update to change things, but if my browser would start to behave erratically and I knew I haven’t updated it for a while, it would have send me on a wild goose chase for other things that I might have done that make my Firefox crash or whatever. It wouldn’t have occurred to me that some change to my settings was pushed without my knowledge.
We can only hope FF devs will use that power responsibly. It can be a nice feature or a complete nightmare. And I for one would very much welcome some kind of pushed notification about that (‘hey, there was a problem with this and that, so we changed that and this; here’s how you can revert the change if needed’).