Mozilla: Use-after-free working with events in FontFace objects (MFSA 2017-06)

A use-after-free can occur when events are fired for a "FontFace" object after the object has been already been destroyed while working with fonts. This results in a potentially exploitable crash. This vulnerability affects Firefox 52, Firefox ESR 45.8, Thunderbird 52, and Thunderbird 45.8...