The built-in Password Manager can only deal with web addresses. What you could do is to use a form filling add-on. They can be set up the way you want.

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for what you want.

A hostname is just the part between https:// and the next / character.

The logins.json file has a field called formSubmitURL but Firefox doesn't store the full path, which would have been a good way to resolve this issue. It's just the hostname of the form submit action (usually but not always the same site).

I think an add-on or third party password manager will be needed for this site.

If you can suggest add-ons this would be good.

With my add-on I can also modify the "submit prefix", which I assume is the formSubmitURL, but when I set this to the one used at http://example.com/adminboard (it is "http://example.com/adminboard"), Firefox does still auto-fill my password into http://example.com/customuserlogin.

People mention a variety of commercial and free programs here, and I don't have a specific recommendation. There are many password managers, some using local storage and some using cloud storage, some with close browser integration, and some that use the clipboard. I haven't tried any recently, although it might be time...

Here's one example: KeePass is FOSS and can run on Linux using the .Net-emulating Mono package. More info, sometimes conflicting:

• https://codeyarns.com/2015/12/01/how-to-use-keefox-in-ubuntu/
• http://askubuntu.com/questions/291002/how-to-integrate-keypass2-and-firefox-using-keefox-passifox-in-ubuntu-14-04-16
• http://keepass.info/
• Firefox integration: https://addons.mozilla.org/firefox/addon/keefox/