The saved info for forms feature in firefox has been driving me a bit nutty. I love it but I also occasionally need to design forms and have filled my forms with transient dummy data, which firefox then remembers. This is mixed with form-fill data which I do want to keep, so I don't want to delete the entire form fill cache.
Easy solution. Use your keyboard to highlight the unwanted entry and hit Shift-DEL keys. Away it goes :)