I noticed a little bit ago that the form autocomplete behavior was not working as I expected, but I hadn’t quite figured out why until recently. For whatever reason, I often start typing in a form field, wait for the autocomplete list to appear, press down arrow to pick the appropriate autocomplete selection, and then press right arrow. This works beautifully in the location bar: it positions the cursor at the end of the completed selection and lets you type in the rest of the path or press Enter to go to the location. Unfortunately, when you press right arrow in the form autocomplete, it just closes the autocomplete box and leaves you with nothing other than the characters you originally typed.
I’m happy to report that somebody noticed it, reported it in bug 283777 and fixed it. The fix is coming in the next Firefox 1.0.x update along with other good fixes.