They had an implementation that did not require any accounts. They deemed that it's too confusing for users* to scan some QR codes (or type a bunch of characters), so they went with an account-based system.

*) Or whatever the real reason was. It was about time Mozilla started to have management issues so I absolutely won't be surprised if the goal was to "become an ecosystem" or "gather a userbase" (or whatever is management speak for forcing people to sign up).