I'm curious, why not just use Firefox Multi-Account Container[1] and setup Facebook and Google to open in its own container[3]?
The way I see Facebook Container (as an outside who have been using Multi-Account Container for a while) is that it tried to ride on the Delete Facebook wave few months ago to make people aware of Multi-Account Container functionality, so these Google Container/Reddit Container/etc. always seems weird to me.
[1]: https://addons.mozilla.org/en-US/firefox/addon/multi-account...