for Thunderbird, base startup event on the last restored window's map.

Unfortunately there is not equivalent "sessionstore-windows-restored" in TB
yet.  Many events related to windows are issued before windows are fully
realized. "mail-startup-done"/"mail-tabs-session-restored" are issued by *each*
window. "console-api-log-event" is not reliable and can be issued at relatively
early stage on heavy configs. So that the best solution is probably to count
restored windows ourselves...
remove experimental fade feature for AMO review
tune pref tooltip for mail_change_trigger