I'm unable to test FF4, but I appear to see a similar thing in FF8 (Vista). It looks like FF is attempting to anti-alias the jaggies?! And it is failing somewhat along the transparent edge. Whilst this seems to work OK on the light (yellow to white) side, on the other (white to yellow - transparent) side a dark line appears.
Looking more closely at a page of CSS shapes in FF8, that I did a while back, I see similar issues. Depending on the colour used it is more noticeable. With yellow it is very noticeable!
I'm guessing that earlier (than FF4) browsers did not attempt to anti-alias these diagonals?
One workaround appears to be to not use transparent, but explicitly specify the same colour as the background!