It is a core feature. What isn't a core feature is the UI to use it. That's because Mozilla wanted anyone to be able to create a plugin using the underlying code and not force a specific plugin on them. An example of a plugin using the underlying code is Sidebery, a tree style tabs alternative (a much better one if you ask me). You can use multi-account containers without using the multi account containers plugin.
It is a core feature. What isn't a core feature is the UI to use it. That's because Mozilla wanted anyone to be able to create a plugin using the underlying code and not force a specific plugin on them. An example of a plugin using the underlying code is Sidebery, a tree style tabs alternative (a much better one if you ask me). You can use multi-account containers without using the multi account containers plugin.
Aaah okay! That makes a lot of sense actually :D