All the heavy lifting for containers is built-in. It's just the mapping of tabs to containers, the creation/disposal of containers and UI parts that's managed through the extensions because users do have different preferences and use-cases in that area. E.g. they may want automatic rule-driven assignment of domains to containers except when either blah blah holds or when manually overridden.

The built-in minimal and completely manual UI can be enabled with privacy.userContext.enabled = true

I would love a way to sync my sites defined in containers, so that site A opens in the correct container on all my computers running firefox.

I would really love tabgroups to magically start working again, but I have been making due with containers. (and it does have some nice features)

Containers used to be the other way around, baked into Firefox and then moved to an extension last year. It's better that a power user feature is not a part of the default installation.

I am not sure if that'll be the case. The Multi Account Container (https://github.com/mozilla/multi-account-containers) seems like it's dead. No new commits since end of March and according to https://github.com/mozilla/multi-account-containers/issues/3..., it's only going to get bug fixes and maintenance patches. No new development.

I have a feeling that they are relying on extension developers to add features/functionality.

You can turn it on, it's called First Party Isolation.