Hmm, where is that wrong text coming from?

One possibility is that the login form is in a frame hosted on a different server. I've seen this on some help sites that frame a Zendesk form, for example. If you right-click the login area, does "This Frame" appear on the menu? If so, you can either using the "View Frame Info" item to see what server it's on or use "Open Frame in New Tab" to work with the frame as a separate page (sometimes this does not work).

Another possibility is that it is encoded in a cookie or storage item saved by the site. Not a good practice, but I suppose it's possible. You could try flushing that out using this method (may dump other customizations and multi-factor tokens):

While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. Firefox will ask you to confirm; go ahead and confirm.

Then try reloading the page. Assuming you are returned to the sign-in form, is there anything in the password field?

>> If you right-click the login area, does "This Frame" appear on the menu?

It does.

>>using the "View Frame Info" item to see what server it's on

Thanks, this finally revealed that the actual login page was at id.tinypass.com, and that's what it's stored under in Firefox password manager.

Much appreciated!