jamesravnikar, speaking as a user, I would suspect that only the shortened URL is saved because that is all that is needed? That when you visit "a.com/b/c" Firefox will simply check whether any logins have been saved for "a.com", and if they have, then they will be offered?

If you visit the login page via a bookmark, or by entering the full address in the address bar, then does Firefox offer you the stored login?

(You might need to enter any primary password or other authorisation that you have enabled first, and you may also need to click on the username or password field and select the stored login from the dropdown list, if it isn't autofilled. Or alternatively, you can right-click one of these fields and select the "Use Saved Password" submenu)

Firefox only saves the origin (protocol and hostname; first part of the URL) and not the full path name when saving a login and also when creating exceptions, so you can't save a login with the same username and different password for various paths on the same server (you can on subdomains).

You would have to use a Password Manager extension or external Password Manager that offer this feature.