This is one of the core plugins I use (in addition to Ublock Origin).

With the account sync, containers are also remembered across different computers as well, so I don't have to set up the containers on every new laptop or reinstall of firefox. Sometimes I mistakenly open a website in the wrong container because I don't always map every single website to a container, but most of the time it's a non-issue.

I'm surprised this is still an add-on and not a core Firefox feature.