All I know is that one of them has a /v2/ subpath on its URL, and the other has nothing.
Oh, and calls work in Firefox one the /v2/ one. That’s an important difference.
What I really don’t know is why they kept pestering me for months to make sure my browser supports the new version (where they know what my browser is, and only published enough requirements to tell IE won’t work) while they only changed stuff that makes it work better on it.