Also keep in mind although Firefox containers is made by Mozilla it's a plugin that you have install, which was very counter intuitive when I first wanted to use it.
The plugin is technically just a management interface for functionality that is already in the browser. FF just doesn't surface it by default, because devs think it would scare casual users.
That's not the only reason. The other reason is that although it's pretty obvious to any one person how it should work, no two people's ideas are the same. Depending on how you are using containers, the answer to a number of questions can be completely opposite to someone else's usage. The theory was that Firefox would provide the underlying functionality, and specialized addons would specialize it to a style of usage.
In practice, it feels like that has hurt adoption. You can't pave the cowpaths if the cows aren't venturing out of the corner of the field.
Honestly the implementation seems half baked, it's very hard to move sites to new containers, there is no list of sites with containers they belong to, etc. I don't know why you'd add a feature in the first place and then decide to hide it for vast majority of users instead of going the extra mile of making it user friendly and intuitive.
Probably because it's a bit of a difficult thing to explain to users in the first place. Instead, by providing just "primitives", they opened the doors to focused add-ons (like the one that isolates Facebook, effectively the first large-scale deployment that used it) without spending too much time on a feature that folks might have found difficult and might result in increased support issues. The status quo is that power-users can make it work, add-ons can use it if they wish, and the overwhelming majority of users can carry on happily ignoring its existence; not a terrible trade-off. But I agree that the management add-on could be better.
I believe Apple is doing something similar with their virtualization API - it's there, some folks can use it, but they don't want to surface it to the masses.
I mean relative to the effort put into building this feature making the UI intuitive shouldn't be a huge task. Think when FF released "tabs" for the first, super intuitive and instant adoption. I wonder if FF is short on UI/UX contributors.
Basically it just lets you extend how many roles/accounts you can switch between in the AWS console UI.