Bug 1055355 - Give content editable fields ARIA textbox roles.

Not sure what you mean by that, I don't see any element with that ID. Basically that markup uses the role 'group' to logically group all of the buttons + entry that is the recipients field. The labelledby attribute uses the localized name of the "to" field.

When a screen reader enters that region by swiping or exploring, they hear "to", so they know that is the recipients input.