Fantastic idea, besides the UI needed it should be manageable to implement.

The UI is already partially there. You can choose which extensions are enabled in private Windows. It's a matter of extending that to containers.

Yeah, and also I’d like it to ask me what container to open for a website sometimes, out of remembered ones. E.g. I have a website I use in just two containers (out of 15), and I like it to ask each time I open a new instance, so I won’t have to choose out of all my instances, just those two.

What's your use case for that? Just wondering.

For example, testing website functionality without an ad blocker.

Honey / Rakuten are very useful extensions, but I don't want them having access to every site I visit.

In Chrome I use profiles and set up those extensions on my "shopping" profile only.

In Firefox I use containers, but can't isolate these extensions the same way

You can do it with OS-level user accounts.

You can't with containers, they're running on top of a current profile.

Why not use about:profiles?

Because I can't run different profiles in the same window. Also, the profiles don't share bookmarks.