Rated 4 out of 5 stars
I managed to get it to work with Thunderbird on my Macbook Pro M1. As my intended sync was with my iCloud account, I had to get apple app specific password.
For the benefit of others, you need to login with your appleid and your *app specific password* and NOT your appleid pw.
It work like a charm with my iCloud calendar and contacts BUT not the task (Apple's Reminders). The funny thing is that once I connected with my iCloud account, I can see my lists of reminders on iCloud. After successfully syncing my calendar and contacts, when I select the 2 lists I have on iCloud Reminders and do a sync, the 2 lists disappeared from the list of other calendars etc. No syncing took place (of course).
BTW, in Thunderbird, it seems I cannot create tasks under the iCloud calendar. Anyone can shed light on this limitation? (I am assuming this is a limitation, for some reasons)
If you select the icould option to setup your icoud account I specifically print that info about app password.