There are only two add-ons needed to sync iCloud contacts and Calendars with Thunderbird: Provider for CalDAV & CardDAV and TbSync. Together, both of these add-ons provide everything necessary to enable iCloud Contact and Calendar syncing for Thunderbird.
Setup of both these add-ons is straightforward. In Thunderbird, navigate to Tools -> Add-on Preferences -> TbSync. In the TbSync Account Manager, click Account Actions -> Add Account -> CalDAV & CardDAV. Select iCloud for the account type and follow the wizard to completion.
Once finished, enable the account, and select which calendars and contacts to sync. Be sure to set a synchronization time as well.
Another useful add-on is CardBook. CardBook’s iCloud sync is independent from TbSync. TbSync puts iCloud contacts in the Thunderbird address book, while CardBook uses its own address book which opens in its own tab.