Tab not working properly in firefox

[piebe]

Open the standard example plunker in firefox. Tab to the second box, use arrows to highlight, press tab. Expected behaviour is that the next field gets the focus. Instead the focus is on a hidden field and the user has to press tab two more times to focus on the next field.

Issue seems related to the number of choices that are offered. For example when only showing 3 options, tab works properly. (also shift-tab works in all cases)