suggestions:
1- organizing the containers in folders and sub-folders structure: I don't use it as much because I manage many accounts, and it would be nice to have the option to create folders. For example: you would have a folder called "Others accounts" and in it there is two folders "Managed businesses" and "family." Inside "Managed businesses" folder you would have business1, business2 etc.

2- adding description to containers and viewing them when hovering over them

3- have more custom control on colors and icons of containers, let the community make icons which could be added within the add-on

4- domain specific containers, example FACEBOOK.com

5- having two check marks; ☑ Always open this website/URL (Facebook.com) with this container (My Facebook) ☑ Except when I am in a different container.

Meaning if you made a container for a family member Gmail, Facebook, or their Other accounts; For example your mother, father, or technologically illiterate girlfriend accounts then you would be able to protect your privacy on ANY site not just Facebook, and manage other people or your other accounts.


Edit:

6- when long pressed the search bar or the arrow key in the Address bar button you can search or navigate to a site with a specific container

This tool is really helpful, as it helps me use different accounts for different services, without having to login and logout of accounts

As part of my work, I am regularly logging in and out of the same site with different usernames. Because each container holds its cookies separately, I'm able to run multiple instances of the site, which cuts back on my login/logout time and drastically improves my efficiency.

Firefox user for life because of this add-on.

I think there are a couple of additional features that could be added going forward, like default container and switch to container. But so far its brilliant and I get alot of use out of it, between personal and work accounts

very nice extension: it allows to easily manage multiple accounts (I use this to separate my work and home stuff)

Excelente, permite separar cookies y almacenamiento manteniendo una experiencia unificada de navegador e historial.

oui

An awesome addon that makes it easy to isolate work tabs and personal tabs containers allowing you to login into the same site with a different Identity in each container. The ability to give default containers for a site is also super useful.
Areas that require improvement:
Make it easy to edit list of domains to default container associations

Сама идея приложения хороша, но однажды вылетел браузер и приложение после запуска забыло все пароли и настройки, какую вкладку в каком контейнере открывать, а сегодня я отключил все плагины в Firefox, включая Мультиконтейнер, (нужно было проверить работу браузера без них), а после включения увидел, что Мультиконтейнер плагин забыл все мои вкладки (а их было настроено около 20-30), настроенные с таким трудом и временем, и показывает те 4 вкладки по умолчанию, которые были сразу после его установки. Синхронизация постоянно включена, но не помогла.

Great Add-on!!
Nice to tidy up and organize Tabs. I like the possibility to easily separate websites, which shouldn't get in touch.

Just needs some additional configuration-tools +sync!
-rearranging containers, inclusive default (as I need it), is a must have! (I need default Tabs at the end)
-Pages in a container+settings(like always open in container X) have to be synced to other firefox-devices!!
-also to android
-more symbols colors
(- Advanced Options)
These point are not given at the time I reviewed it!

..would be nearly perfect, improves my experience a lot!
I really want to give it 5 Stars!!!

Very useful. I like the new features (always open, hide container, etc.)

Very nice, still missing some features – that would make the Containers a solid alternative to multiple Firefox profiles/Chrome's „people/supervised users“. Especially that Firefox forces a single instance (I consider this a deep architectural bug by itself).
1. Synchronization via Firefox sync.
2. Setting a default container for a domain is clumsy arnd unclear.
3. A better integration with Password Manager (..., History, Bookmarks, Addon Manager, Preferences,...) would be very nice.
4. A possibility to reopen the current URL in a different container is a must!
5. I'd expect that container tabs inherit from the „default“. Currently, the „default“ is just an ordinary container without any added value, just a bit different menu/toolbars/uncolored tabs, which is misleading.
6. Protect containers with a password? Especially if the Password Manager would be containerized.

Also it is not clear, which Addon is „THE containers“. There are many extensions on AMO with very similar functionality.

Очень удобное расширение. Как разработчик, часто приходится тестировать сайты, и контейнеры позволяют не запутаться между тестовым/продакшн сервером, а также помогают хранить раздельные аккаунты на разных сервисах одновременно. Короче, очень удобно)

Houd verschillende zaken makkelijk gescheiden binnen dezelfde browser sessie. Werkt heel goed en makkelijk.
Keep different stuff separated within the same browser session. It works like a charm.

Missing points:
- Configurable container icons (not only more, but the possibility to add our own icons)
- More colors (or visual patterns! like dashed, dotted...) options
- Default container: it should be possible to open (by default) all the new tabs into a default container if no specific match-by-domain is possible.
- External apps integration: It should be more easy to open links from other applications (like Slack, or Thunderbird) into specific containers
- Navigation History integration: It should exist a way to navigate through the navigation history filtering by container
- Temporary containers, something like this: https://addons.mozilla.org/es/firefox/addon/temporary-containers/?src=hp-dl-promo

Great
no need profile switcher again

Really useful!