Hi DickGil, it might take a few steps to track it down.

Firefox's Password Manager

If it's only the username and not the password, it's probably not this, but just in case, you can check your saved logins here:

• Mac: "3-bar" menu button (or Firefox menu) > Preferences
• Windows: "3-bar" menu button (or Tools menu) > Options
• Linux: "3-bar" menu button (or Edit menu) > Preferences
• Any system: type or paste about:preferences into the address bar and press Enter/Return to load it

In the left column, click "Privacy & Security".

Then on the right side, scroll down to the Logins & Passwords section.

Click the "Saved Logins" button. Here you can view/edit passwords ("Show Passwords" button) or delete logins completely.

Firefox's Form History

When you click in the username field, and either type the first character or press the down arrow on the keyboard, does the username show up in a little drop-down? Try arrowing down to that item and pressing Shift+Delete (on Windows, just Delete) to see whether you can remove it that way.

Website Cookie

Sites may set cookies with information that helps them customize pages when you return. To remove the site's cookies (save any pending work first), you can call up the Page Info dialog. While viewing a page on the site, try either:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info
• (menu bar) Tools > Page Info

Along the top of the dialog, click Security, then on that panel, click "Clear Cookies and Site Data".

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

Then try reloading the page. Does the customization disappear?

that you can also use "Forget About This Site".

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox from this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox