Migrating from Sunbird to Lightning
From MozillaZine Knowledge Base
Now that the upcoming release 1.0 of Sunbird, the free standing version of the Mozilla Calendar, has been announced as "the last one ever" (unless, of course, it finds a group of enthusiastic supporters to maintain it, the way the Mozilla Suite did and became SeaMonkey), Sunbird users will sooner or later have to migrate to Lightning (the same Calendar application, but packaged as an "extension" for Thunderbird or SeaMonkey). The aim of this article is to make the transition as painless as possible.
Before you migrate
Since Sunbird, Thunderbird and SeaMonkey all have different locations for their profiles, you will have to "export" to somewhere on your hard disk any calendar which has been living in your Sunbird profile. This is not hard, you can still use Sunbird (as long as it is supported) after you do it, even use it to change events and/or tasks in the relocated calendar(s), but it is a necessary preliminary step.
I recommend the ICS format for the "exported" calendars on your hard disk. Repeat this until you have "exported" all the calendars in your profile. (If you are also using remote calendars and/or calendars on your HD but not in your profile, you don't need to export them since they already are at an "application-agnostic" location.)
Lightning can be installed in Thunderbird or SeaMonkey from the Lightning homepage. The installation procedure is the same as for any other extension:
Importing your calendars
After you restart the mailer with Lightning installed, click the Calendar button near the right end of the tab bar in the Thunderbird or SeaMonkey 3-pane Mail/News window. This opens a tab whose contents look like your familiar Sunbird. Use File → Open → Calendar... to tell Lightning where your calendars are (on your HD and/or on the net).
If you had been using the "Provider for Google Calendar" extension in Sunbird
The same gdata-provider.xpi can also be used in Thunderbird or SeaMonkey, provided of course that you have Lightning installed. You can install both extensions at the same time so only one restart is necessary.