Rated 5 out of 5 stars
This is a bit long, but there's a lot to explain.
TLDR version: TbSync rocks with Office 365 calendars!
I was using some other Thunderbird (TB) add-ons which support Exchange, and which I'd started having problems with, to the point that I removed TB from my Linux workstations in frustration, and thought I was stuck with the Office 365 (O365) web client. After a few weeks of dissatisfaction with that solution, I decided to revert to IMAP on TB for mail, and accept that for my work O365 invites, I'd have to use the web client. I did a clean install of TB, and noticed in the body of some invites in my inbox that I could opt for calendar invites to be sent in iCal format. Thinking that might help, I followed the link in the invite body and enabled it. Well, after that, I noticed that calendar invites now had a completely blank body. What a bummer.
Well, it is Friday afternoon, and nothing productive happens on Friday afternoon, right? So, wanting to be at least semi-productive, I decided to do a bit of googling to determine if I could see the contents of the invite in the email, thinking I would know if it was worth switching to the web client to deal with it. I found a post that mentioned installing the Lightning calendar add-on might help on that front, and surprisingly (at least to me), it fixed that little quirk -- I could now see the invitation details in the email body. Success!
Did I mention it was Friday afternoon? I really didn't have the energy to start working on stuff I really should be working on, so I decided to do a quick check to see if there was an add-on that might be able to deal with the .ics attachment. I don't remember the exact search I did, but I found TbSync, and thought it was worth a try. (I thought I remembered reading that ActiveSync wouldn't work with O365). I installed TbSync, added my O365 work account, and it works! I see all my events, there isn't any weirdness with repeating events that I had noticed with the other extension, and I am back in business. This Friday afternoon seems to have been quite productive indeed.
It's early days, and I need to test more before I can fully sign off on this as having solved my issues with Exchange calendaring in TB, but I am hopeful. Thanks for creating this!