Fix conditional check where alarms snoozed for exactly 60 minutes were never fired off.
Use nsiTimer component for hourly thread/alert slider.

This seems more stable and reliable than the setTimeout/setInterval.
Hopefully this will fix the issues where some users would report that alarms stopped firing after some number of days.

See
https://forums.mozilla.org/addons/viewtopic.php?f=7&t=17033
Merge remote-tracking branch 'origin/master'