Memory corruption in IPC FilePickerShownCallback

When creating a callback over IPC for showing the File Picker window, multiple of the same callbacks could have been created at a time and eventually all simultaneously destroyed as soon as one of the callbacks finished. This could have led to a use-after-free causing a potentially exploitable crash. This vulnerability affects Firefox < 117, Firefox ESR < 102.15, Firefox ESR < 115.2, Thunderbird < 102.15, and Thunderbird < 115.2.