The value of the security.insecure_password.ui.enabled preference determines whether or not Firefox warns you about non-secure login pages.
- enable the feature and be warned about non-secure login pages by setting this pref to true
- disable the feature to be warned about non-secure login pages by setting this pref to false
- security.insecure_password.ui.enabled = false
You can disable the warning when you type in a name/password field by setting this pref to false.
- security.insecure_field_warning.contextual.enabled = false
There is signon.autofillForms.http for HTTP sites as an addition to the general signon.autofillForms pref to disable or enable filling a form automatically.
Source: How can I disable Insecure password warning in Firefox? | Firefox Support Forum | Mozilla Support