Very nicely done. It allows to compartmentalize my online life: one tab for Google products, one for Facebook, one for Amazon, one for banking, and the default for the rest.
Idea for a feature: have a per-container type option specifying whether new links opened from that container should open in the same container type or the default container (or even, specify in which container to open links).

very helpful tool when managing multiple accounts in one browser

This is the single most used add-on I have. My job requires logging into multiple sites with different accounts, and Containers makes it such a breeze. Brilliant add-on!

This is an absolutely brilliant extension, the setup is super simple, you can create as many custom containers as you like. Personally I've kept all facebook owned sites in the facebook container, I've then got one for all google owned sites, one for twitter and then general ones for the different types of sites I visit, so tech, news, general, personal, banking. Basically you create the containers, create a new tab in the container, go to a site and then tell the extension to always open that site in that container. Bingo, you're done. And, the longer you have it, the more sites you find yourself adding to each container. This coupled with Privacy Badger and HTTPS everywhere and I know less of my information is leaking out. I will say, it's sad that the online world has now got to a point with tracking that we have to do this sort of shit. But, if it forces the likes of Google, Facebook et. al. to change their ways then I'm happy to do it.

I really enjoy the ability to isolate sites with containers. Works great if you want to be logged into multiple accounts on the same website. Two or more gmail accounts open at the same time.

This functionality is wonderful when doing web development. Testers/developers can be logged into all the website roles at the same time if desired. Great when used in combination with https://addons.mozilla.org/en-US/firefox/addon/temporary-containers/

Best solution I found for managing multiple AWS accounts simultaneously.