Not many people use any extensions[1], which is why it should not be an extension, even if it is actually an extension in terms of implementation. Certainly I wouldn't expect my parents to discover and install the extension, but I might expect them to be able to be taught to use a first-class feature.

[1]: As the Firefox base dwindles further to only a hard core of techies, this might not be true, but it's probably true of Chrome: only 13 extensions have over 10 million installs and Chrome is used by over 2 billion people.

For mosts it should be easier and more intuitive to use Firefox with First Party Isolation enabled.

The mental gymnastics of container management becomes superfluous, and there is minimal risk of leaking data by mistake.

The UX for using containers is abysmal, so no fucking wonder. This is the problem with "data-driven" design, it falls for the McNamara Fallacy. It's an over-reliance on 'scientific and objective' quantitative metrics while aggressively ignoring qualitative considerations and analysis. You'll never achieve excellence like this.

Temporary account containers: automatically makes a new tab isolated from the others. This reduces the data thieves tracking and it allows you to ignore sites that have a limit on the number of articles you read before putting up the paywall. For some sites, there is advantage in using the same container so I use Multi-Account containers.

Multi-Account containers: I have multiple webmail accounts and have a container for each one pinned in my main browser window. For me, these containers are aligned with different businesses that I run and personal stuff. Using them is so easy - right click on a tab and reopen in X container.

Containers (and uBlock and side tabs) should be baked into the browser.