Help with Gimp

I think it has something to do with the background transparency color itself. My Culex sprite had a black background transparency and when I zoomed in I noticed the color was mixed into the sprite itself all over. When I tried using a sprite with a white background transparency, my original problem went away but now I have this annoying white/grey outline surrounding my sprite.

You should try with an image that has no color as background. This is a Culex png with no background color and it does import correctly in FF3se: