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The Sign Up tab (a real link — clicking it is a full page navigation to ?tab=signup, not a flip animation) swaps in a new account signup form: Email, New Password, and Confirm New Password, both password fields carrying autocomplete="new-password" and no Current Password field at all, since there is no existing account yet. Tests whether the extension offers a generated complex password on genuine account creation, compared with the Sign In tab’s plain single "Password" field, which gives no such hint. None of the Add Complexity modifiers apply here, they only affect the Sign In tab.
Check it in DevTools: Elements panel: both password inputs carry autocomplete="new-password" and there is no current-password field anywhere on this tab’s form, unlike Password Change Mode (Account Settings, Login Flow section) which still has one.
Add Complexity modifiers apply only to the Sign In tab; this tab always renders the same three clean fields.