To the best of my knowledge there is no uniform protocol for task synchronization, unlike mail, contact and calendars. So your only hope is a specialized add-on. I am aware of ones for exchange activsync ([https://addons.thunderbird.net/en-us/thunderbird/addon/tbsync/ TBSYNC]) and Google calendar and tasks ([https://addons.thunderbird.net/en-us/thunderbird/addon/provider-for-google-calendar/?src=ss Provider for Google calendar]), but nothing for Icloud.