I remember the good old times when Mozilla had a project named Chrome (yes) to (if my memory serves me correct) make building apps with gecko easier.
edit: Probably misremembering, now that I searched for it. Yes "chrome" was (and still is?) used to describe the non-webview parts of the FF but apparently I totally made up the project part.
Because IIRC in old Mozilla Chrome was a project name that was the UI layer of Firefox. Chromeless basically just mean Firefox without UI for other applications to built on top.
edit: Probably misremembering, now that I searched for it. Yes "chrome" was (and still is?) used to describe the non-webview parts of the FF but apparently I totally made up the project part.