By default Firefox only allows users to delete all the form history entries, however if you only want to selectively delete form entries, there is no easy workaround. Form history entries are basically the entries that you have already typed in while using Firefox, with the help of form history, you will see auto-complete suggestions while filling out forms, making it easier for the user.
An Firefox add-on called Form History Control, lets you view and manage form history entries with ease.
With the help of Form History Control add-on you can manage all the form history entries, remove a single entry, edit and entry, insert a new entry and more.
You can also use the add-on to search for the entries in your form history.
A cleanup section provides the ability to store multiple fieldname and/or value combinations for easy removal of unwanted form history entries with a single click.
It is also possible to export/import the form history as well as the cleanup configuration, allowing you to exchange data between multiple Firefox configurations.