Doesn't an .ics invitation or event notification arrive as an attachment that can be saved? Once saved, I'd have thought double-clicking it would open the application associated with it.

No, Microsoft Outlook sends the .ics as a multi-part/alternative section, not as an attachment. Thunderbird displays the text/plain or text/html portion and ignores the existence of the text/calendar variant.

Lightning allows Thunderbird to interpret this style of communication. SO I suggest your choices are few.

My personal choice would be export your apple calendar as an ICS and import it to lightning and go with what works, instead of tilting at windmills. Microsoft are not going to change.

Alternatively you could retain a programmer to write you an add-on that manages the Outlook generated content as you choose. But that would be expensive compared to just using what you already have for free.

Matt, it is not "free" to switch calendar programs and lose the current functionality I have (which includes syncing between devices).

I fear that Thunderbird is becoming a "walled garden," and that saddens me. It's been a great tool in windows, linux, and OSX environments for well over a decade. I don't, however, want it to try to become the environment, or force replacement of other tools with which I'm happy.

gdinwiddie said

Matt, it is not "free" to switch calendar programs and lose the current functionality I have (which includes syncing between devices). I fear that Thunderbird is becoming a "walled garden," and that saddens me. It's been a great tool in windows, linux, and OSX environments for well over a decade. I don't, however, want it to try to become the environment, or force replacement of other tools with which I'm happy.

Walled garden, your entirely free to remove lightning. if you get ICS files your more than free to include them in your external program like you always have. So I have no idea where this walled garden rot comes from. Outlook has changed, Lightning has adapted. As Thunderbird never had and is unlikely to ever have some sort of handler for this non standard way Microsoft chooses to send invitations, I am suggesting you go with something that works.

Lightning synchronizes with devices using DAV, but it does not synchronize using a plug like Apple devices do. But then that is Apples walled garden of proprietary plug types at work. So instead of throwing rocks at Thunderbird for not doing what you want, examine your options. Apples calendar is clearly of almost Zero use as you do not have ICS files to pass to it. I do not know is apples mail program can talk to it's calendar program without ICS files. Perhaps it can. I really have n idea.

What I do know is apples calendar can synchronize nothing better than Lightning. Lightning can even synchronize to apple online service, once you spend time on the apple support forums locating the hidden links to the calkdav URL buried and well hidden in Apples settings files. Apparently they use caldav internally, but chose to hide the fact.

If you want to talk walled gardens, I am happy to do so. But I will start at Cupertino, not Mountain View. Personally I am more interested in actual solutions to problem, and you have one on the table.

Matt, I see you are a moderator. I suppose that means that people posting here can expect to be told they want the wrong thing. I'm sorry I bothered you.