I opened Thunderbird to find that the folders I had created under the Inbox had all disappeared (I had simply added folders for the year in question...2021, 2020, 2019 etc and I place emails I wish to save in the year they were created). I am still receiving emails as normal.

The suggestion is that Kaspersky may be involved in some way.

Any suggestions would be most appreciated as to how I get my folders back.

Cheers

@DakotaC47 Welcome.

I don’t encounter your issue , can you please  check Kaspersky > More Tools > Reports ?

OK. I discovered that if I create a folder under Inbox with the same name as a previous folder (I used a folder called ‘Alex’), then added a message in the Alex folder and then pressed the newly appeared ‘down arrow’, to the left of the Inbox, all of the missing folders suddenly appeared.

Don’t know why. It just did. So...does not look like Kaspersky was the problem after all…

Cheers