Hello John.

You can delete all form autocomplete entries by going into Tools > Clear Recent History... and ticking "Form & Search History" (don't forget the timespan at the top too). This will, however, delete all entries for all fields. If you just want to delete the entries for a specific field type, simple double-click the field (you'll see that a dropdown appears with the first few entries shown) and click Delete. Hold it down until their all gone, or use the arrow keys to select which ones in particular you want to delete (in theory, hovering the mouse over them will do it too).

I hope this helps.

See also: Control whether Firefox automatically fills in forms http://kb.mozillazine.org/Deleting_autocomplete_entries