tn, have you saved passwords for that site in the Firefox password manager? If you have, then this might be the source of the suggestions.

If not, then have you tried right-clicking an entry for this site in history and selecting "Forget About This Site"? (But read the message that appears about what will and won't be removed.)

Yes, I did have a saved password for that site but that was the first entry I deleted along with all browsing history, forms data, and cookies related to that website.

When I go to the login page for that site now, and click in the user name dialog box, it still shows two usernames I have used previously for that site (and others). Highlighting a browing history entry for that site and selecting "forget this site" does not change this behavior.

The mystery is where FF is getting these suggestions from.

tn, are the suggested logins ones that are still saved within the password manager?

If you search for them there, are they listed as logins for other sites tht may be in some way related to the site in question?

I often use the same user name for several different online forums. Some of these are on related topics, but they each have a different domain so no excuse for FF to confuse them. The forum/site that sparked this question is shopsmith.com. This forum and some others I visit have been created using the PHPBB forum software, but this should not matter. Even if all of these forums were built using a single shared database of users, FF would have to be getting the username suggestions from the site itself rather than locally stored data. I suppose FF could query, "I have this unique identifier (IP, MAC, GUID), what usernames have used this in the past?", but I doubt it works that way.

tn, when you cleared form history did you use the "Clear Recent History" option from the top History menu, ensuring that the item "Saved form info" was checked and the time range was set to "Everything"?

Do you hav any add-ons that work with either passwords or form fields? Have you tried Troubleshoot Mode from the top Help menu?

If so, one way to definitively prove the matter would be to create a new profile in about:profiles and, without making any changes, visit the site and see if it happens. If it does not then, with Firefox closed, copy the files "logins.json" and "key4.db" from your current profile folder to the test one, open Firefox and visit the site again in the test profile. (there are options in about:profiles for opening new Firefox windows with any of the registered profiles, opening their folders on the hard drive, or setting one of them as the default profile.)

If it now does, then this would prove that Firefox is judging that one of the sites stored in the password manager is sufficiently similar to the specific site (the two files mentioned above store the local password manager information).

Sorry, that is all I can think of to try. If any one knows other tests or has another explanation then hopefully they will post here.