Browsers shouldn't require accounts, and thankfully Firefox doesn't. But having a way to synchronize my passwords across all my devices is pretty important.
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).
Sure, strictly speaking, of course there were accounts as in "database records". However, users gave no details about themselves, and haven't signed up for anything. No long-term credentials, just a rendezvous point.
You used to just open two Firefoxes, start sync setup on the one, copy (or scan) a code to another - boom, done, they're talking to each other.
*) 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).