Mozilla calendar programs (Sunbird, Lightning) do not store their data in ics files natively. They use an SQLite database (the 'storage.sdb' file inside the profile). This means that calendar data cannot be indexed directly.

To get Recoll to index calendar data, you need to export it to an ics file. This can be done manually, from the application menus, or, by installing the Automatic Export extension.

The extension can be configured to export the data when exiting the program, or at regular time intervals. You can even set up a command to be executed after the export. If you are not using real time indexing, this can usefully be recollindex.

In Tools→Add Ons→Automatic Export preferences, in the Start an application after export subpanel, set Path of application to '/usr/[local/]bin/recollindex' and Parameters of application to something like -i;/home/me/path/to/nameofexportedcal.ics

This will ensure that the calendar is indexed every time it is exported (this is not necessary though, you can let the next batch indexing pass take care of it).

It may happen that the exported data has some syntax errors which will prevent indexing with the rclics filter which was distributed up to Recoll 1.13.04 (included). You may get an updated filter from the Recoll download page.