This direcotry stores all your profile's persistant data, including bookmarks, history, passwords, preferences, extensions, custom themes, website data, security settings, and browsing sessions.
about:profiles and hit Enter.about:profiles page will tell you which is currently in use.
There is a bug in Flatpack versions of Mozilla that prevents the root directory of profiles from being created in the default location.
This makes installing UserChrome, UserContent, Sine, and Sine mods less automated and generally more complicated, or sometimes not at all possible.
You can fix this by instead installing the Tar version of Zen browser, in which profile directories work as expected.