Outlook calendars have always required an addon. See https://support.mozilla.org/en-US/questions/1557280

I have solved the problem by reverting to Thunderbird ESR hich allowws these addons

Hi, I was looking for info about syncing my outlook.com calendar with Thunderbird because I've recently had issues with it and had to remove it, and then couldn't find a way to re-add it - after finding this thread I now know why!

I looked at the Thunderbird desktop roadmap, it says that full exchange support is "Active" - does that mean it's working in Thunderbird or that it's being actively worked on? The roadmap isn't clear, there's no explanation of what the development categories mean: https://roadmaps.thunderbird.net/en-US/desktop/

Hi all, Very nice that someone has said my post is a chosen solution, but what do the roadmaps actually mean!? 😭