I presume that you have found the relevant controls in Thunderbird and you have set them appropriately?

Is it an IMAP-connected account? If so, your email provider may have a retention policy that is removing the messages. If this is the case and you can't adjust it, then you may need to set this account to use the Trash folder in a different account that doesn't have an automated emptying regime. The special Local Folders account could be used.

I do struggle to understand the logic of deleting messages and expecting them to remain in the Trash folder indefinitely. Why delete something you may want to keep?

I'm not at all savvy about installing, uninstalling, reinstalling, IMAP, pop3, defaulting or modifying. My Thunderbird was installed by a Tech in India who apparently wasn't exactly familiar with how to do it. Are you saying that if I can "modify back to pop3," Thunderbird will stop deleting from Trash? Can you tell me how to do that or should I call someone? Also, am I able to retrieve the lost emails? Once when my whole Inbox disappeared, the Tech was able to find them (of course, he didn't share the "how" of that).