A basic version of this feature is part of Multi-Account Containers since version 6.1.2. My extension is more complete (colored icons, edge cases such as folders and private browsing mode), but if you don't need those, then you can enable the feature in Multi-Account Containers as follows:

1. Right-click on the extension button of Multi-Account Containers.
2. Choose "Manage Extension".
3. Click on "Preferences"
4. Click on "Enable Bookmark Menus".
5. Confirm the permission request for access to your bookmarks.

If you are interested on the (technical) background of this feature in MAC, see https://github.com/mozilla/multi-account-containers/pull/1537