Looking at what the html input field in toolkit's autocomplete inherits, the xpfe one misses a few attributes. I noted this when investigating why there is no placeholder text in the places/bookmarks panel tags field.
Created attachment 513622 [details] [diff] [review]
Add some more attributes
placeholder was what I was looking for in the first place, since that makes us show the placeholder in places/bookmarks panel. Haven't tested the others.