I have had people tell me this works, but I haven't found any trick to make it work for me. As far as I can see, Thunderbird will import from an ics file or can export to one, but will not use it as an interactive "back end".

I use the cloud, aka Google Calendar via Caldav.

Could you run your own CalDav server?

The way you do it is a bit counter-intuitive:

• when setting up a new calendar you choose network calendar
• in the location field where you normally have http/https url you put a local path, say file:///home/yourusername/tmp.ics (linux specific, but you can use the same file: protocol on windows)

You also want to make sure all external modifications to the file are done when Thunderbird is not running as this may cause issues.

Thank you! Here in Windows, and using Dropbox, the pathname I entered by hand was automatically sanitized to:

file:///C:/Users/yourusername/Dropbox/calendar/calendar.ics

I think the correct formulation of this URI is where I have got it wrong in the past. I wasn't confident about how to join up the local path name with the file:// protocol declaration. The URI used on my Linux (LMDE) system is rather more rational. ;-)

It's now working very nicely, with a shared ics file, as you can see, on my Dropbox account. As Merike comments, I'd be cautious about what may happen if you're able to run several instances of Thunderbird concurrently. I suspect the last one to write will overwrite changes made by others.

Note that if you do use Dropbox, you need the name of the local version of the file, not the link to the copy on the Dropbox server that they offer you.