It turns out that the migration or infobar to enable the built-in lightning version does not always work as it should; apologies for that!
To enable lightning/calendar if this situation applies to you, do the following:
- If you still have an old version of lightning installed (no longer supported/compatible), uninstall that.
- Navigate to the folder where you installed FossaMail (normally C:\Program Files\FossaMail)
- In it, go to the folder "distribution", and in that folder, go to "extensions".
(Please note: do NOT go directly to "extensions", go to "distribution" first). - There should be a single folder there, "{e2fda1a4-762b-4020-b5ad-a41df1933103}", open it.
- The folder you are now in has a bunch of folders and files in it. Select all items, and add them to a zipped/compressed folder - name this zip something useful, e.g. "lightning38.zip" and save it in an easily accessible location, like "My Documents".
- Rename the zip file you just created to "lightning38.xpi". Confirm that you want to change the file extension if your O.S. warns you about changing the file type.
- In FossaMail, go to the add-ons manager, and from the gear menu, select "install from file"
- Point it to the lightning38.xpi file you just created, and let it install -- Done!