>privacy enabling features of firefox that are served as extensions like the containers feature can suddenly and without warning stop working.

containers is a core feature of the browser. The addon (Firefox Multi-Account Containers) only adds some niceties like an addon button to start containers, and auto-assigning sites to containers. Without the addon, you can still manage containers by going to about:preferences#containers and open a tab in a given container by long pressing the new tab button.

Any idea why Mozilla has chosen to implement it this way?

Nobody thinks that Mozilla’s remote kill switch for extensions is an error. The reason I accept it is because I don’t want to be personally responsible for every extension I might install. Nobody has time for that. Let someone who does this stuff for a living take care of it.

I agree with that sentiment, It would just be nice to be able to turn of that kill switch, you know?