Enable Built-in Containers Feature per jingyu9575

Installing this extension will enable Firefox's built-in containers feature.

This can also be done by setting "privacy.userContext.enabled" to true in about:config, but the preference can be reset by Firefox if the last container extension is disabled. If you are debugging another container extension and need to disable it temporarily, this extension can be useful to keep the built-in containers on and avoid container data loss.