Do you still have the backup? If a full backup, the folders should be there. However, if the backup was run while Thunderbird was running, then it's possible that folders are missing in the backup. If the folders are not in the backup, they're gone.

Thank you, David. I don't know if the backup was created while Thunderbird was running. But I do have it. Is there a way to check whether the missing folders (and, I believe, missing address book, too) are indeed in the backup?

Okay, here would be where to look. I'm assuming in my example that old drive appears as drive D. - open Windows Explorer with Thunderbird NOT running. - d:\users\<yourid>\appdata\roaming\thunderbird\profiles.ini is a text file that shows name of the profile. - the profile would be in

 d:\users\<yourid>\appdata\roaming\thunderbird\profiles\<profilename>\Mail and/or Imapmail. Local folders would be in the Mail folder.

Thanks again. Finally I managed to check the profile backup and turns out that at least some of the missing folders are indeed there. But I cannot see them when I run Thunderbird (with this profile). I've tried checking if I just need to subscribe but they're not on the subscriptions list either. What should I do to bring them back?

Are they folders within the account, or local folders? Are they in the active profile, or in the backup profile?

I've found the missing folders in the backup profile within one of the email accounts (a gmail one), not local folders. Unlike other folders that have been successfully restored from the backup, about 3-5 of them did not make it to the new Thunderbird installation, only in the backup profile. What would I need to do to recover the missing folders?