Absolutely great add-on!
Should be a built-in (or added by default) feature.
It's my No#1 tool for keeping services separated from each other, e.g. Facebook separately contained, News websites with their tracking cookies separately contained, other user accounts/profiles in various other websites and/or services w/o any association between them all.
It's like having multiple, persistent "Private Browsing / Anonymous" tabs with the ability to define a persistent default container for every website (e.g. clicking a Facebook link will always open up in a pre-defined Facebook container tab).
I love it and use it all the time!

Very good idea, needs more work, but nevertheless extremely useful in 2019s internet

Please, please, PLEASE allow us to backup and restore settings with Firefox Sync. Batch categorization of sites in the bookmarks manager without the subsequent "Open this site in your assigned container?" notification would be great as well. This will be the best extension ever if you can get these two things done!

This is a great extension for separating work more effectively across multiple accounts on the web, all for different purposes. You can choose to create your own containers if the default options are not enough. I would like to see an option to add custom Favicons for each container, and RGB color choice for the default colorful icons. The most important change I would like to see is that when I set a website to open in a container by default, I should be able to reopen that site in another container without it defaulting back to the choice I made. Keep up the good work.

really needs to be a built in feature, love it!

Great concept but not useable for me. For example I use DuckDuckGo "bangs" a lot. For visiting Twitter I type !tw in to DuckDuckGo. I want to automatically switch containers when I visit Twitter however with a redirect (like with duckduckgo bangs) I have an "empty" tab in the default container and a new one with twitter which is pretty annoying. I hope this get's improved in the future.

Annoying