ReminderFox I really want to get my life under control - and I have been trying lots of Calendar and TODO list programs. At first sight yesterday this is the best. Just the way the little knot appears in bottom right corner and moving cursor over it THE MAJOR items appear.
Note you can have it in Thunderbird and Firefox - you just need to go into options and adjust the File used so as the same in both - full instructions atReminderFox
There are lots of features but for quick calendar and major ToDo stuff this is good
You can have it RF running in both apps but just in one lot of options turn off Alerts else the Alarms come up double