How would this help with CSRF? The new feature only applies when someone intentionally clears their cookies.

Not how I read it. To me it says Facebook.com cookies set through site A are separate from those stored through site B. Even if you never clear, fb cookies would no longer be a single cookie to link these, but separate.

The jar is the key new thing, not the emptying.

The approach to the cookie jar isn't new in Firefox 91; that's Total Cookie Protection, added in Firefox 86: https://blog.mozilla.org/security/2021/02/23/total-cookie-pr...

Delete the word “new”, emphasis on “key”.