Use these steps to remove saved (form) data from a drop-down list:

1. click the (empty) input field on the web page to open the drop-down list
2. highlight an entry in the drop-down list
3. press the Delete key (on Mac: Shift+Delete) to remove it.

• http://kb.mozillazine.org/Deleting_autocomplete_entries

• http://kb.mozillazine.org/Password_Manager
• https://support.mozilla.org/kb/password-manager-remember-delete-change-passwords

Thanks! it works!

Can you tell me where exactly this information get saved?

If it has been saved as form data and not as a name/password in the Password Manager then this data has been stored in the formhistory.sqlite file in the Firefox profile folder.

• http://kb.mozillazine.org/Profile_folder_-_Firefox

You can use this button to go to the Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)