Firefox now isolates third-party cookies by default for all users. This means that you only need containers if you want to do something like:
- Logging in to multiple accounts on the same site
- Browsing sites that use cookies to limit how many articles you can read
- Having private tabs in the same window as your regular tabs
If you want to remove the extension but not lose the functionality, it's possible to use about:config to manually enable containers by setting
privacy.userContext.enabled and privacy.userContext.ui.enabled to true.
You can then edit your containers from about:preferences#containers