"default-release" is your default profile for the Stable release of Firefox.

"dev-edition-default" is the default profile for Firefox Developer Edition, so I'm assuming you had that installed at some point.

"default" to my understanding is only there for legacy reasons. It used to be the default Firefox profile way back in the day, but now that there's so many different Firefox versions (Stable, Beta, Nightly, & Developer Edition), using the same default profile for all of them across editions would cause corruption and a lot of issues. So that's why Mozilla moved to "default-release" for the default profile on Stable Firefox, similarly Developer Edition makes its own profile, "dev-edition-default", as would Nightly, Beta, and so on. I'm not really sure why "default" is still included, its probably safe to delete, at the very least it can be safely ignored.

root directory

Where your settings, bookmarks, passwords, etc are stored.

local directory

Where your cookies, website local storage and other temporary non-profile data is stored.