Review for ReminderFox by zer0rez
Rated 5 out of 5 stars
This light yet extremely robust calendar/reminder addon is deceptively powerful and is especially helpful if you work at your computer. Integrates nicely with thunderbird as well.
There is a brief video overview of it here:
http://www.youtube.com/watch?v=as9XUOFccGA
To create your own collections, you must have a Mozilla Add-ons account.