Glass still safe after boil-off?
While @Brendan's answer is the correct one as it comes directly from the manufacturer, the risk with glass and other ceramics is not that it will start to leach chemicals into the water, but rather that the glass has been subjected to heat in an abnormal and uneven fashion (the heat is at the bottom and localized, the top will be coolish).

This uneven heating could potentially lead to stresses on the glass that are not visible to the naked eye, but have resulted in microfractures or general weakening of the glass structure. As a result the kettle could potentially collapse/shatter, especially if filled to a high level and boiled. This would result in large volumes of boiling water being released and could result in burns from direct contact with the water and from steam.