lets start with the most obvious. Firefox and Thunderbird are related by shared code internally. But neither uses the other to run. So Thunderbird is not accessed using Firefox.

Is the account IMAP or POP? Can the account receive new mail? What occurred immediately before these folders went missing?

Folders appearing or disappearing are not related to indexing. They can be related to anti virus programs, internet connectivity, mail server connectivity, file corruption and a few other things. But indexing is not really one of them.

Thanks Matt, I appreciate your reply. I'm thinking it might be an indexing problem because I found an elaborate post that suggests deleting the associated .msf file and having Thunderbird rebuild it. I haven't tried this and I haven't spoken to the owner to have her at least do a search on *.msf to see if the indexing file(s) exist.

Problem solved! The missing folders were accidentally drag-dropped into other folders. I found 2018 inside 2019 and 2016 inside Sent Items.

I was sort of loath to suggest that one. Folks get somewhat strange sometimes when you tell them they did it.

BTW, right click a folder, select properties and then hit the repair button rebuilds MSF files. No need to go poking around in the file system where Windows goes out of it's way to hide them.

However folder issues are very unlikely to be indexing. More likely deleting the foldertree.json file will fix that, or actually subscribing to the folders if the account is IMAP..