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.