KeePassXC is a password manager that I use to fill-in passwords in Firefox (with official KeePassXC plugin). All websites that have login fields will get populated with the KeePassXC icon, which can be clicked to fill-in the password.
The only exception is InvoiceNinja. It does not get recognized as a login field. Is there something non-standard about the InvoiceNinja login page that would cause this?