You can very quickly eliminate the mistyped entries with the down cursor key and use of "Del" key on the selected item ("Shift+Del" on a Mac). So you would just have the ones you want. You do not need to type the first letter you can double-click within the entry box to see all items whether to use or to delete.

That does not answer what you asked. If you only want to suppress that, I don't think that would be possible with Firefox alone, but I also would not think that you would want to suppress all autoform entry because of that.