Starting around Firefox 107/108, I started noticing that Firefox was suggesting usernames to me when attempting to enter website credentials. In the "Privacy & Security" section of settings, I have all "Logins and Passwords" saving and autofill turned off. I have all "Forms and Autofill" turned off. So to have Firefox start offering suggestions for login usernames was surprising.
Checking "Saved Logins..." - I have no saved login credentials in Firefox. Again, very odd that Firefox is suggesting login usernames to me.
I ended up turning off Syncing of my data across devices. I refreshed the Firefox browser on several devices. I eventually deleted my profile on several devices. Deleting profiles temporarily resolved the issue... until I started logging into some of my regular websites again.
I eventually found that Firefox is storing usernames as "form History" - this itself does not seem correct, but that may be something to do with how individual websites are coded.
Either way, now that I knew how it was saving these logins, I went to find out how to turn it off. I found this: https://support.mozilla.org/en-US/kb/control-whether-firefox-automatically-fills-forms
The setting I need to turn off is "Remember search and form history". I don't like this. I want to keep my search history - I reference it nearly every day. I do not want Firefox saving, suggesting, or auto-filling forms or my login credentials. I consider this something completely separate from my search history.
I think the "Remember search" history should be separate from the "Remember form" history.