When logging in, is there anyway to make it so Fluther (or Facebook) doesn't automatically fill in my whole user name when I just type the letter "D"?

If I interpret your question right, you are referring to the “Dutchess_III” username for example which appears under the username box when you type “D”.

To delete it, point your pointer right at the username when it appears when you type “D” and press “delete”. It will go.

Hope that’s the answer to your question

I think you’re talking about the “autofill” or “autocomplete” feature that most browsers have. This page tells you how to delete it for most browsers and operating systems.