I believe they'll add that soon, but they haven't yet because of either: 1) The code base is not production ready yet, and they don't want an issue to come up

I believe they'll add that soon, but they haven't yet because of either: 1) The code base is not production ready yet, and they don't want an issue to come up which leaks users passwords 2) (More likely) They intend to add support for LockBox, their extension that replaces mediocre password support with a full-fledged password manager. And they need the Add-On support before they can include that. So most probably you'll see this along with add-on support come in the next few months