Review for TbSync by Simon Matthews
Rated 5 out of 5 stars
It works very well.
Since Office365 moved to OAuth2 ("Modern Authentication"), I had to remove the existing Office 365 account and re-add it. This worked and now my calendars are syncing once again.
Removing calendars did not work: I needed to remove the account in the TBSync account manager and re-add it.
To create your own collections, you must have a Mozilla Add-ons account.