Did you open the Calendar tab, then Events & Tasks, Import..., then select 'Outlook comma separated values' (*.csv) in the file type filter? Press Alt if the Menu Bar with Events & Tasks is hidden.

Thank you for your reply sfhowes. Yes I did all that and get the Error Message: "Cannot import from C:\Users\***\Tasks.CSV. There are no importable items in this file."

Actually I have just discovered from another question:

https://support.mozilla.org/gl/questions/1290595

that the format generated in a csv file exported from Outlook is somehow not a true csv and would need altering.

I created 2 new tasks in TB, exported them from TB to an ics file. I deleted the 2 new tasks, then imported the ics file into TB, but nothing was imported. This suggests the TB import function is not working at all - not even to re-import files that were exported from the same program?

I see the same issue with importing a csv that was exported from TB. The csv has no information besides the headers. But it should work by exporting from Outlook in ics format, as described in the link you posted.

Unfortunately neither my old Outlook 2010 nor my new Outlook (which came with Office 365) will export in ics format. Only CSV and pst formats.

I'm not an Outlook user, but I see references to saving, not exporting, a calendar to iCal ics format:

https://www.msoutlook.info/question/export-or-backup-complete-calendar-as-ics-file

https://superuser.com/questions/202389/exporting-outlook-2010-calendar

Thank you again very much for your reply. In fact I had previoulsy also tried to import the calendar in ics format having saved it from Outlook, but it does not contain any tasks (only appointments) or at least only appointments are imported into Thunderbird.

The new release of TB has very satisfactorily resolved the problem of TB only importing Recurring Events into the TB Calendar, but not Non recurring ones. I successfully imported all Outlook Appointments via a file exported from Outlook 2010 in .ics format - a big thank you. However, this process unfortunately does not import Tasks, it only imports Calendar Events. It is not possible to export Tasks into a .ics format file from Outlook 2010. The only options are to export to a CSV (either DOS or Windows format), Access, Excel, Outlook Data File (.pst), or Tab Separated Values (either DOS or Windows format).

When I select Events & Tasks in TB, then import, there are only 2 formats acceptable, .ics and CSV. However when I try to import those Tasks exported from Outlook 2010 in CSV format, TB still says it cannot find any data to import. Perhaps it is possible for you to fix this issue too?

TB 91.3.1 is now available and includes this fix:

"Importing an ICS file with TODO items failed, bug 1727938."

Does this affect importing tasks?