Não há caixa de seleção "Salvar senha" ou algo semelhante.
I played around with about:config but got nowhere.
Você pode usar a extensão Editor de senhas salvas para sites onde o Firefox não oferece para salvar o nome de usuário / senha:
About this Add-on
This extension allows you to enter data into the
Password Manager database instead of relying on Firefox, Thunderbird,
SeaMonkey, etc., to do it, as well as making changes to existing
entries. For web forms, the typical usage is as follows:
- Navigate to the page containing the login form.
- Right-click on one of its fields and select from among "Save login info" (to create a new entry), "Edit login info", and "Delete
login info".
- If another popup appears, select the username for the entry you want to operate on.
- If adding a new entry, everything except the username and password should be filled in automatically. Fill in anything that is
missing or incorrect.
- If adding or editing an entry, reload the page or return later. If the form doesn't get autofilled, most likely the page is asking the
browser not to autofill it by declaring autocomplete="off". Try
clicking twice on the username field and selecting from the dropdown
list. There is a video demonstrating this workaround.
This add-on also adds commands to the Password Manager window, which
it makes accessible through Tools > Saved Passwords as well as a
toolbar button.
Fonte Editor de senhas salvas