I'm afraid you're both wrong, containers are an add-on, profiles are a core functionality.
That person does not want something new, they just want to use their existing workflow, which is possible, but inconvenient due to bad UX and ugly UI. These are their words, not mine.
As a user of both containers and profiles in Firefox I have to agree with them. The UX needs to improve and the UI needs to be prettyfied.
